When I am fighting another country, particularly when they're the aggressor, They will often come close to 0 war score and then propose a surrender, with terms that are not the terms I want (all money). When I reject them, they get 20 war score back, and it takes a few more turns for me to get back to 0. This happens 1-2 times before they stop.

I don't think this is a bad system. It makes sense that if they surrender and I reject it, their country may think it's unjust and want to continue fighting. But, I don't understand why sometimes they do this and sometimes they don't. 

If I was on the other side, I would just continue to offer surrender until my score depletes.

This doesn't happen, they usually stop after 2 times. I just think I'm missing something fundamental.

I can't be certain, but it may be due to how much war support they are losing per turn. I think (not confirm) that if they are just losing a few points per turn, they will offer surrender. But if they are losing more (for example 1 city +) then they don't have the ability to offer surrender and so it gets to the forced stage. There may be a cooldown on how often they can offer surrender, so if they are losing support too fast, they cannot offer again. Or it may have something to do with whether they think they can turn around the situation or not - if they have no army left, they know they can't win.

I would be happy to be corrected by someone who actually knows! Thank you!