Referencing this comment:
Is it justifiable to downvote a question if the OP has accepted an answer right away?
Personally no it's not acceptable.
First off it's going against what voting is meant to be for where in this case someone is voting because of the Asker's actions to an answer and not because of the question. this would be like downvoting a question because the Asker accepted someone else's answer and didn't upvote yours1 or downvoting another's answer just because theirs were accepted and yours wasn't. Questions should be voted based off from their own merits such as
Secondly as per the Help Center on What to do when you get an answer to a question
- Accept it. As the asker, you have a special privilege: you may accept the answer that you believe is the best solution to your problem.
the point of the accept vote is to show that the Asker believes the answer they have accepted is the best answer that solved their problem. when an answer is accepted this doesn't prevent other answers being posted or edited and unlike up/downvotes you can remove it at any point (up/downvotes get "locked" after so long unless the post as been edited).
More importantly an accepted answer doesn't lock up/downvotes as such an accepted answer might be right for the asker but not right for someone else. using a Stack Overflow and my personal opinion if an Answerer posts a code dump of the Asker's code but fixed with no explination (ie. "try this [CODE DUMP]") and the Asker accepts it, the accept vote would not stay my mouse when i downvote it because while the code solves the Asker's problem it is useless for anyone else who doesn't have the exact same code.
Also there is a Badge for having a better answer than the accepted one
Highest scoring answer that outscored an accepted answer with score of more than 10 by more than 2x. This badge can be awarded multiple times.
and i have seen plenty of times on Stack Overflow where the accepted answer has been < 0 with another answer being more highly scored.
1: this is ofcause for situations where it's easy to determine this like if your answer score is +0/-0 (meaning no one has voted on it)
To downvote the question because OP accepted an answer quickly? As far as I'm concerned; no, that's not the right thing to do. That said, strictly speaking it's not against the rules; except for things like serial or targetted voting, votes are basically for each user to do with as they please. Except for cases of obvious vote abuse, it's also not something that a moderator has any power at all to reverse.
I have been known to point out more or less forcefully myself that questions with accepted answers can receive less attention from the community, which in turn could cause the OP to miss out on better answers. But that's just a matter of whether people read the question; questions that look like they've been concluded seem, all else equal, less likely to draw the attention of any particular user.
Question votes are about the question: whether it's clearly written, useful, etc. They are not a way to indicate disagreement with OP's choice of answer to accept. If you disagree with the accepted answer, same as with any other answer, then vote or comment on the answer. If you feel you can provide a better answer, or even just provide a different perspective (which seems to be not uncommon on Writing), then write your own answer.
That said, I agree with Monica that I haven't seen this as being a major issue on the site. Might it happen? Sure. Is it possible that some particular user is doing it more systematically? Of course. Is it a rampant problem? Highly doubtful.