Spell of return, when can the AI cast it?

[Question]

Ive noticed that sometimes when taking a fortress city, the AI doesn't cast the spell of return, they just get defeated and all their cities get brown flags...what causes this?

[Catwalk]

I think it's to do with mana reserves, possibly gold reserves as well.

[Asfex]

If combat finish with mana non-zero,and this wizard have other city,then it start cast spell of return.if he has only outposts at the finish spell of return,the game crush.