Why will other empires break deals for no reason?

My last save file ended terribly and it’s partially due to my greatest ally breaking our defense pact. Why did they do it? We got along fine, we’d set up all the other pacts and deals and such, so I don’t get why they’d break the pact (and just before I planned to go to war to get my old territory back)

Is it an ideological thing? They were religious zealots and my empire were war hungry, so I don’t think they should’ve gotten along well in the first place.