I was banned 30 days + 2 years of signature ban due some posts of more than 3 years ago.... No big deal (I recognize my error) but I don't get why someone started to scrap the whole post history of an user and report to moderators 
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Maybe just to increase the % of good rep.
.Maybe just to increase the % of good rep.
Perpetuating Quicksy's fallacies LOL
You got caught because you have a shitload of plagiarised posts so your chances of getting caught were correspondingly very high. I don't think anybody targets specific users, I certainly don't because that would be a stupid waste of time - the effort that is needed to check 10k posts of one user (and probably end up with nothing) is much better spent to check 10k posts that are likely to be actual plagiarism (and probably end up with 1-2k bans).
tl;dr: Don't plagiarise.
I know, that was long time ago
. Why report it now? No big deal because I don't care about signature, just trying to understand the reason).
What do you mean with "Perpetuating Quicksy's fallacies LOL"?
I agree with you when you say "don't plagiarise".