AI keeps resigning way too easily?

So my friends and I have been playing super casually for a while, but have decided to actually try and get better. We have started upping the difficulty and can beat 1 to 1 moderate now. The problem is the AI seems to resign almost immediately after we attack their base sometimes. We will go in, destroy maybe their town center, and they resign. They did this with 4 castles once. Is this a bug? Or what is going on? It's really not satisfying to work hard and build an army just to have them give up immediately.