Is that normal? Should I take some action on it? How to figure out who is it?
You can't find out who it is as all votes are kept anonymous.
It's not normal or welcome for someone to go round down- (or indeed up-) voting posts from specific users. You should be voting based on the content. There are scripts in place to detect what we call serial voting where there are a large number of votes from one user to another over a short period of time (where "large" and "small" are numbers not known to anyone outside the development team), but it's possible that some votes will fall outside these values and go undetected.
If you spot something you think is suspicious then you should flag one of your posts for moderator attention using the "other" flag and explain that you think you're a victim of vote abuse.
There are tools available to moderators where we (I'm a mod elsewhere) can spot voting trends and this will confirm (or not) if there is targeted voting going on. If that's the case we can ask the team for those votes to be invalidated and then send the perpetrator a message and even suspend them if the vote abuse is serious enough.