How to find the reason for someone giving you a -2 for a previously answered question that had been given 10 points?
I pose a frame challenge.
Nobody gave you +10 points, nor did they give you -2 points. What happened is that people used the ratings system built into the site. This is not dissimilar from other sites with user-generated content that ask users to rate content.
The site used those ratings to give you +10 points in an effort to encourage the kind of content people want to see, and also to give you -2 points in an effort to discourage the kind of content people don't wan't to see.
The question you mean to ask is:
Why did one person dislike a post that another person liked?
and you can hopefully recognize the futility of posing it as an abstract question.
Any user with 15 points can vote up on any question or answer they want, and any user with 125 points can vote down on any question or answer they want. The default is that no reason is given. To communicate specific feedback on the post a comment would be used instead. The voting provides a sort of rating system, inevitably with a lot of noise, but on the whole appears to be useful, especially if more people vote more often.
Want more upvotes on the site? You can do your part by voting up more on useful posts. As for our own posts, we are unfortunately psychologically programmed to be affected even by the noise, but votes are not necessarily even intended to communicate anything to the poster, but rather are a user's use of their rating privilege on the site.
See also this answer.
You can take odd mystery downvotes as an opportunity to re-read your answer and see if it can be improved.
If no improvements are obvious, you could assume it was a slip of the mouse, and forget it.
The other part of the question is +10 vs -2. Each downvote decreases your rep by 2, but upvotes increase your rep by 10.