Antagonism spikes?

Hi, ive played the game for like 200 hours and i feel like i understand the mechanics but theres a thing that comes up every now and then i just dont understand:

I was playing as Florence and making sure in the peace treaty screen that i dont get beyond 50 antagonism with too many people. After my last war with siena, all was well for about 10 years. Then suddenly without an event, i get a huge coalition against me out of nowhere. I checked my antagonism and it had skyrocketed to 120 with my immediate neighbours (which had 50-60 before) and was beyond 50 for many more. This has happened several times across several campaigns and its making the game unplayable because i cant plan expansion. Also after this happened, even taking a single province now suddenly increases antagonism by much more.

Anyone know if im missing something?

Unrelated question:

Do you really have to "annex" seperatist rebels? It seems stupid youd get antagonism for putting down a rebellion in lands that have been part of your country for 20 years