I came to know that there is a script to detect serial down-voting but as it is the case always for any script, there is a set-threshold, and if a careful person somehow figures that out (for example, by trial and error method), he can do Serial Down-voting by carefully waiting and then down vote in a measured way or can use different accounts or friends in the same forum.
Today my 8 posts were down-voted as a part of serial voting in 2 succession (in this week total 12 posts in 3 successions).
When I faced this type of problem in the past (happened 4-6 times), moderator (or the system?) responded by undoing that (4 times), but recently they have turned a blind eye , also, the person who did this, didn't face any consequences (like getting banned), so he is keep doing this.
On the other hand up-vote will always meet with severe punishment, my hundreds of points have been reduced in (2 successions) and I was banned. It looks like serial voting is crime if it is a upvote, but if it is downvote, MSE is happy to let this bullying happen!