It could also be a single conquered city. Annex a large city (15+ pop) and you will immediately see a swing of -20 or more
That effect I am prepared for, and obviously the timing is under the player's control. Sometimes my happiness swings so much that rebels appear in a single turn. I am sure it is not, but it really feels random and arbitrary sometimes.
But overall I would STRONGLY advice against picking an ideology with the intend to switch to another one later.
Actually, I completely agree, and I see I miss-typed in my previous reply. I don't pick ideologies
planning to switch. It's just that the penalties associated with switching, losing 1-2 SP (but only ones that you got for free anyway) and a couple turns of anarchy, are actually quite mild. I would argue that either the half-price factories or the 6 foreign legions are easily worth that. But I think it is more fun to play to your preferred ideology. So choose your preferred Ideology, but if Order or Freedom, pick up perks you get to keep should you need to switch later. Win-Win.
Does Autocracy have any SP with lasting benefits like free units or half-cost factories?
Most Perks that really help you winning the game are t3-perks and by switching to another ideology you reset your progress to get them.
The new ideology
also has tier 3 perks that are almost just as good, and maybe better, for 3 out of 4 VC. You are not completely resetting progress, just losing one or two SP you got for free. If you had a bunch, you still have a bunch, and you get to pick them all at once, which can be a significant advantage. Plus maybe your were keeping your VC options open when you first picked an ideology, but by the time you switched you have a path picked out.