dick1982: my potahtoe can't run it so meeeh. guess i'll watch the let's play videos and fap to doujins.
the_importer: Didn't even know you could have a Reputation in negative here, guess there must be a reason for that.
I think the artist formerly known as GameRager (now...gamezranker or something - sorry) is currently in the "lead" at -2172.
"Reason" on the other hand is a bit in the eye of the beholder - the short answer is gog created a system that was well-intended but is prone to abuse esp. once people figured out how to maximize abusing it, and even in its best days was probably only a proxy for anything meaningful.
Any post that is low rated gets you up to -5 per day IIRC, whereas you get +1 for posting per day, and I want to say +2 per day for a high rated post. So it is actually easier to go down than up if a small group or even one person willing to waste the time making multiple accounts wishes to make it so.
So if you consistently get low-rated posts either naturally or more likely as above where its coordinated, you can descend quite fast. So negative reps certainly happen, and isn't necessarily a good tell about the quality of someone's posts.
It's one reason many giveaways have moved away from rep as a requirement, and why regulars know to either ignore it entirely, treat rep as a source of amusement/scorn, or at least read it with a large grain of salt.