I asked my first 2 questions on this site, and they were fairly just riddle puzzles. For some reason, after I accepted the right answer, downvotes started coming.
Why is this so?
Usually, downvotes don't come because of accepting an answer. It might be a coincidence, that more people saw your riddle and thought it was a poor one after the answer was posted/accepted than before.
Sometimes, a question does get heavily downvoted after an answer is accepted, because people feel that the accepted answer makes no sense, and therefore the riddle itself makes no sense if that was the intended answer. I've seen this happen a couple of times: a puzzle seems good and gets upvotes, then the OP accepts a crazy answer and the upvotes rapidly turn to downvotes.
In the case of at least one of your puzzles, maybe the downvotes came because a very short riddle is often a poor one because many possible answers can fit so few clues, or maybe it's because people felt, as this comment indicates, that the answer you accepted didn't fit well with the riddle.
For further reading on improving your riddles to be better received, I suggest:
Welcome to the site!
When speaking of old posts, accepting an answer might draw some attention to your post from the +2 link in your profile, and the +15 link in the answerer's profile. However, as your posts are fairly new, I don't think the links had anything to do with the seemingly additional interaction on your posts.
Here are a few things you might benefit from noting:
For your first post...
In my opinion, one reason it wasn't well-received is due to how it appears to have been made without enough effort. The accepted answer didn't make things better, as it can be perceived as obvious, making it seem less than a proper puzzle.
By "obvious", I mean like "If X is the answer, then wouldn't both Y and Z be valid too?". I can think of quite a few alternative common words to that puzzle that would work pretty much like the accepted answer.
For your second puzzle...
I feel that the quality is an improvement compared to your first puzzle, as it strikes as a puzzle rather than a random trivia question. But upon accepting an answer, users felt disappointed at how the intended answer didn't really match the puzzle's description. There are comments here pointing out the issues.
Also, as you can probably tell from the fact that there are currently 4 different answers on that puzzle, the puzzle was perceived as too vague, which links to being made with low effort.
All in all, please don't get discouraged from participating on this site by a few downvotes, as it happens to the best of us. Just learn from those posts (not necessarily yours) that aren't well-received, and you'll be alright :)