There are people who find it very enjoyable to destroy someone's reputation (for basically the same reason that there are people for whom murder is such a turn-on that they cannot stop themselves from doing it till they get caught).
Also, there might be ways to profit or personally benefit from a campaign like this. E.g., one of the authors of this web page might covet one of the titles or jobs Stallman currently holds -- for themselves or for a friend. E.g., Stallman or one of his supporters might be approached in the coming days with an offer: I can make this web page disappear from public view, but it will cost you. Basically any rival has an incentive to try to get you fired and to destroy your reputation.
I suspect that we as a society should adopt the general rule that anonymous attacks on the reputation of a person should be ignored. In the absence of such a rule, anyone can keep on waging campaigns of reputation destruction (in pursuit of getting ahead somehow or of a twisted kind of enjoyment) with little to no cost or risk to themselves. The attack can include lies, and even if the lies are discovered, again there is no cost or consequence to the attacker.