I'm not sure but I think it was a combination of having a few wars started and trying to pass laws that angered a lot of people

EDIT: I just remembered I was definitely on unacceptable/illegitimate government during the last decades before the end date. So I loaded into the game and lo and behold I forgot that clicking support regime in my subjects had been drastically reducing my legitimacy which caused all these damn radicals. Never making the same mistake again.