When you vote to delete an answer after it was accepted by OP, you are basically telling OP
No, you can't accept that answer, even if you already did. You can accept another answer if you feel like.
I wouldn't like that at all if I was the OP, and would consider it very high-handed of the voters if an answer I accepted later got deleted.
While I am not exactly proud of this early answer I wrote 4 months ago, OP found it useful and accepted it 2 days ago.
My answer had scored exactly +2 -2 = 0 in 4 months but got heavily downvoted to +5 -14 = -9 today after it was accepted by OP, and it has collected 2 delete votes.
So voters are basically downvoting it because it was accepted by OP. Or else you had 4 months to cast these downvotes.
@Monica Cellio has rightly pointed out in a comment that
I suspect the (new) bounty brought an old question to lots of people's attention, people who hadn't voted before or hadn't seen all the older answers.
In fact downvoting is OK if users don't like an answer but voting to delete: I see it as an attack on OP's choice and I wouldn't like it to happen to any member's answer I accepted to one of my own questions. Why would anybody want to delete an answer that was found helpful and therefore accepted by the person asking the question!
I am not asking why my answer should be downvoted or deleted. Kindly do not waste your time to explain why. It was obviously a just-OK answer (not a good or great answer) till it was accepted by OP.
We are not talking about the answer itself but the fact that it was accepted. The merits of the answer ought to have nothing to do with it once OP has accepted it. It is very much a case of site etiquette and general procedure whether any answer that was accepted by OP should be deleted unless the question itself got deleted.
I lost 22 points (11 downvotes since being accepted) and gained 20 points (2 upvotes) for a negligible net reputation change plus the rare notoriety of having written a greyed-out -9 score answer that was accepted by OP, but I am only asking about the advisability of delete-voting an accepted answer here.
So is it a good idea to vote to delete an answer after it was accepted by OP?