Yep, that's the AI. It seems very fond of such suicidal attacks using token forces against massively defended targets. Doubly so if you happen to be allied. The very best one I ever had is when my ally England bypassed the lightly defended and vastly rich cities of the Low countries which were held by its enemy to attack Copenhagen by sea, the only city I had in the region, and which was defended by a huge stack of grizzled Crusader troops who I had just taken it with.

I believe the AI is programmed to attack the player regardless of how insane it is in order to keep the challenge up in the late game, since the rational thing for the AI to do when you control half the map would be to avoid war with you at all costs and if you are at war, to desperately beg for peace every turn (which might take some of the fun out of ruthlessly crushing them).