how can my question be voted best answer one day and get a violation email the next?

Well, the thing is an answer can be reported anytime, when it's open and even when it's resolved. Seems to me a troll might of reported you.

If the person who asked the question voted your answer as Best Answer but somebody else reported it as a violation of the YA terms & Conditions.

A best answer can be a violation. Many times someone will ask how to fix a problem such as moving email from one account to another. Someone will answer that Yahoo is terrible in that they do not explain how to do it and they get best answer. Another person who actually tries to answer the question is ignored.

₪ ʎəɿʞɹɐq ₪

That will happen when your answer was not reported before you got best answer. Having best answer does not mean you can't be reported if someone thinks it's a violation. There isn't a time limit on getting reported.

Even best answers can be violations and can be reported after the question is resolved.