The ai for the computer controlled europeans is in the habit of disbanding its soldiers when you attack it
I have seen this happen on several occasions in different circumstances, i go to war and destroy 1 defender and then it puts all its soldiers to work in the colony

You can see from the save games i have attached that in this case the first defender was its ship... why is it defending with its ship when its got a veteran soldier in there? then the turn after it defends with a normal soldier which i think is just a colonist using guns from the warehouse and not an actual soldier

The saves are at the very start of the game but i have seen it do this in the middle of a war when its got several well defended colonys, i attack one and for some reason it drops all its soldiers after the first hit. Stupid, because i would almost certainly have lost against its veteran, but ended up taking its colony

Just a little addition to this, in the attached save you see a french colony with a pioneer standing on it. Im almost certain that its got a hardy pioneer in that colony and yet its using a free colonist to do its pioneering work
The ai probably doesnt work like the player i guess, but it doesnt make sense for it to do this