You say this:
Then I noticed I gained 2 rep from that question and then plus 10. Apparently, at the same time that I accepted that dude's answer, someone canceled their downvote on my question, then upvoted it. So the most likely either the guy whose answer I accepted or somebody else who liked their answer better only upvoted my question once I accepted their answer.
Honestly that whole chunk of your question gave me a headache. Why do you believe some series of events like that has any meaning past what it is: Votes changed based on content changing.
Is this proper conduct? Should people be doing this?
Anyone can vote anyway here for any reason. Votes are an indication of personal opinion.
And the case of some questions, I only upvote a question if I answer the question and the answer is well received. I don’t do this to be political or to hold reputation hostage, but rather I do it because in some cases, a question might be misinterpreted by me, yet I have an answer. I will post an answer and maybe follow up with:
“I wasn’t 100% clear with what your question is, but I tried to answer it as best as possible. Let me know if I understood it correctly.”
If my interpretation of the question is correct and my answer is valid, I will only upvote that question afterwards to assure I am not rewarding a question that is too confusing to truly be well understood.
And in the case of someone coming in after you self-answered, that is normal. If you clearly state you know what you know and your answer “Is what it is…” then someone else can come along and go, “Wait! There is more…”
Nothing that happened on your question is problematic but rather you are seeing the side effect of someone getting a better answer than was expected and someone else looking at that question and going, “Oh! That is a good question…”
Chances are always equal that when a new answer is posted, a new set of eyes sees the question, then sees the answer and simply they are outside of your internalized dramatic interpretation and simply like it as is.
This is a community of people. Getting upset or concerned about one or two upvotes or downvotes is silly. The reason some people have high reps is not because every single thing they post is amazing, but because they continue to post, participate and be a part of the community.
Sure, sometimes someone posts a question that is sure to always get attention and votes, but even people with amazingly well received questions still get downvotes.
As Bobby McFerrin once sang, don’t worry… Be happy…