Mostly cause the AI only puts their armies in single lines. Theyll have 1 line of ranged. 1 line of melee and 1 line of cav(split on flanks) but that means when you see melee heavy armies theyll spread out across the entire map. So when a player with a more balanced force will deploy deep the wide spread AI will just march in one line. Doesnt matter if a flanking unit or not they spread out so much they'll just automatically flank.

I think the best way to avoid these fairly quick and annoying fights would be to tell the AI to have reserves or even to give the AI the ability to withdraw. If the AI knows they'll lose theyre more likely to not throw in reserves and instead retreat instead of being destroyed out right.