How does lichess detect when someone intentionally decreases their rating?

Say for example this user: http://en.lichess.org/@/platedoak

How can the computer classify them as a sandbagger in only 3 games? They may have just resigned the game because they felt they'll lose.

Well, as a sandbagger I'm assuming you mean someone who is a strong player who intentionally keeps their rating low so they can play weaker opponents? If that is so, then the computer could classify them as a sandbagger due to how they play in their games.

"They may have just resigned the game because they felt they'll lose." Obviously not the case here. He resigned right after making <10 first moves without losing anything.

Wait wut i've accidentally decreased my rating on purpose to refresh myself to kinda 'start over'. Didnt know that was against the rules Oops

But i wasnt like feeding one person i was just resigning to random people

@platedoak

That guy only played 4 rated games. It was an obvious mistake to flag him as intentionally decreasing his rating. He was probably just testing the site.
After that he only played casual games, and eventually left lichess.

I call that a system fail.

It was not a system-raised flag. Lichess only automatically marks in cases with high confidence, and with the user platedoak, a moderator marked them as a booster after reviewing him.

"Moderators mark users as cheaters when they deem it appropriate to under the necessary discretion. No feedback or evidence is needed to be given to the public." (copied from: lichess.org/qa/880/why-is-checkyounexttuesday-banned-)

@ConstantinAlex: It was for the better, though! It was not done by the system as Unihedron pointed out. That guy is a well-known sandbagger who exclusively plays the hippo against weaker players. He has dozens of other accounts on lichess.