Questionable AI Decision Making

I'm fairly new to the game and have only rudimentary awareness of diplomatic tactics, but one game I played a day or two ago kind of struck me as dumb. I was playing Noble on Pangaea but it was one of the ones that had narrow peninsulas all around: i was on one to the south, Shaka to the southwest (so my immediate west), Izzy up on a wide one alone Northwest, Giggles and Zara alone on another lobe to the east forming a sort of horseshoe with Monty at the south tip, just across the water from me to the east, with Mansa smack dab in the middle of everything.

So the set up is I'm neighbors with two of the most famous warmongers of the game (though one can't get to me directly) and there is at least one religious fanatic + a couple others that really like religions that might convert the dangerous neighbor. I was actually dicking around in this game, letting the auto production build whatever for a long time, just to see what it thinks it needs to do. I managed to get a few wonders playing as the Industrious Roosevelt, including Great Lighthouse, Colossus AND Masoluem of Mausollos or however you spell that, so I had a decent economy to tech with despite watching the AI building things like triremes/caravels and research instead of any troops whatsoever.

Monty was threatening me left and right, Shaka made a demand or two (that I immediately agreed to, nervously) but none of the pricks on the map were willing to war with any of their neighbors, only those who were across the map from them and blocked off by other civs. Well that does me no good to try to sic Shaka on Monty if he has to go through Mansa and Zara to get there does it? Meanwhile Isabella had converted Mansa to Buddhism and he LOVES her just fine, so nothing there. Zara will attack Gilgamesh, but not Monty or Mansa even though all three border him and Gilgamesh is the same the religion (Judaism) as him! Things are quickly looking grim if I can't bribe Shaka *somewhere* other than at me, as he's roaming around with Chariots and Catapults everywhere but doing nothing for the moment. If Isabella converts him there's no siccing him on anybody else even if I also go Buddhism, as I'm by far his weakest neighbor.

So anyway, I found Confucianism with the Oracle and send my missionary straight to Shaka, knowing if nothing else it will at least buy me a slightly better chance to sic him on somebody else. I can't believe it; Isabella hasn't spread baldies to any city of his yet at all! Yes! He converts next turn and suddenly now he's willing to attack Izzy for cheap, so I send him on his way. In the meantime, Zara decides on his own that there can be only one defender of the Jewish faith and starts stomping Giggles something fierce, all the while Monty is trapped behind Zara's closed border and threatening me ineffectually to no end. I eventually bribed Zara onto him after Monty declared war on his own once and big green put him in his place, wiping him down to 2 cities before I had to bribe him OFF to prevent a capitulation when I saw him approaching Feudalism.

That's not the dumb part. That comes after Shaka marches off his massive 25+unit stack toward Izzy and gets utterly wrecked, losing 1 of his only 4 cities at the moment to her and calling it quits. This peace lasts about 5 turns before I notice that Shaka's red fist has popped up and sure enough, he's massed on my border now with about 20 chariots and 8 or so catapults. He declares the next turn after I spotted it and I go into emergency whip mode, slamming out Macemen from every city in order to absorb as much of that stack as I can in the 3 turns it takes to march on Washington. Shaka apparently bit off a lot more than he thought he could chew, and was not expecting such a huge tech disparity in military or didn't care. I ripped into his stack as it walked to Washington (luckily my dumb auto workers did something helpful for once, and roaded damn near every tile around in my terriotory, so that was nice), he threw it against the 2 maces I had stacked in there and lost about half of it, then immediately called it quits. Ha! Score one for the noob!

The ceasefire takes him back inside his own borders and he marches off in two turns to pick the war back up with Izzy all of his own accord. But I'm pissed now, and stay in war mode massing maces and trebuchets on his border until I'm rocking a stack as big as his was at least, giving him a few turns to march away, and then promptly overrun him with Izzy marching in pillaging right behind, refusing to give up her revenge either this time. Sweet justice! And from then on, with his further 5 cities and more devious diplomacy, I end up uniting with Mansa/Izzy, turning Mansa on Zara who decides he wants to be Christian and bully his neighbors now that they're a different faith now. I proceed to pick the rest of them apart one at time teching up through Knights> Cuirassiers>Cavalry, vassaling each in turn before massing cavalry to overrun Mansa in 4 turns flat. I end up taking an increasing number of cities each turn (1, then 2, then 3...). The best part was none of my vassals did anything except fumble about, and he bribed in Izzy later (of course, because I couldn't drive a wedge between them at all) but my counter attack against her caused her to be willing to capitulate in the same turn when I wiped her stack and took a border city. The hilarious bit was she couldn't because she had just declared that turn, so she had to suffer through losing the entirety of her only ally, all of her military might and another city before she was able to give in.

What I don't understand is how an AI that wasn't plotting on me beforehand could change his mind in the middle of war, cancel it, and decide that after being thrashed once, that taking on an even more advanced target was a good idea. Especially the one that just bribed him a little over 10 turns ago and is his best friend in the world. I mean, I understand that like most warmongers he can plot at Pleased readily but it's like the AI took into no account the fact that my military tech was so much more advanced or the fact that he should hate Izzy more than me because of our shared faith...but noooooo, it just looks at the lying power comparison, decides I'm somehow an easier target, and stabs me in the back despite the fact he was going to (and did) get stomped into oblivion for his trouble. That's pretty dumb to me.