Yeah, theres been a lot of talk of Siam always dominating. It's because of three factors in my opinion:
1. Their Unique ability gives them bonus' to city states, so not only will Siam get more food per city state, it's ai will typically buy more than other civs, getting even greater food and therefore become exponentially power.
Now, even though like you said you had random personalities on, it still doesn't stop Siam from getting the flat bonus, without buying up all the city states.
2. Their Unique unit is extremely powerful, especially when steamrolling other AIs.
3. Their Unique building encourages a lot of science growth, synergized with their UA, they typically have a tech lead over other AIs, and when it comes to AI combat, out-teching is everything, especially when they crack their above Unique unit out quickly and dominate their neighbors.
In short, no other civ has so many synergies, especially at the AI level. Siam is the reason alone I never do random civs, and the one and only civ I never include into a game. I don't mind the challenge, i'm just sick of the Siam superpower every game.