Can't believe no one else thought of infamy as an obvious reason.
I had this once, sudden legitimacy drop, and I used to play around to exactly not hit the infamy limit on my first hungary game.
Could be part of the reason, you should really look out for your infamy limit because it changes constantly.
Thing is, I ignored it because he said "It's at 20" thought that's fine and didn't think much about it. If it's ok, why think about it?
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Some nice infamy reduction methods I used;
-Get Artist advisor, spam cultural decision "Comission Painting".
This will get your cultural tradition up, this works just like in the army and the navy, higher the percentage, better the advisor.
Get it up to 100%, then recruit a.. diplomat I think. Enjoy the -0.30 infamy advisor.
-Casus bellis. Obviously if you take territory without a claim you take a ton of infamy.
This can be ignored in the mid-late game if you have the technology, but for the early game.
If you want early territory, abuse the crap out of the Holy War/Religious Liberation CB.
These will go away I think at 1600 or something, so if you want to gain land easily, use this to your advantage.
Meanwhile, build up your government technology like crazy with land, if you are warmongering. Why?
Absolute Monarchy.
It's badass, if you gain Absolute Monarchy you gain "Imperialism" Casus Belli, you can point on ANYONE on the map with a war threat, and get a 50% infamy reduction, meaning 2 infamy/province. This comes in only at Gov. lvl 27 according to the Wiki, in Death&Taxes it was 33 if I'm correct.
A more viable thing is building an Embassy. These need to be reconstructed often for whatever reason, related to cores or something. But give good -infamy. Also Cabinet is another -1.00 as a National Idea.
Oh, and if you're catholic and worry about excommunication.. The Pope in this game is a total joke, just blind him by sending gifts, he'll let you do whatever you want. The pope in M2TW is much, much more annoying as a christian realm.
Oh, wait. You know this stuff maybe? If yes, that was a waste of characters haha.