According to the user “Noctis Everto” on the Infinity Ward Forums, A new exploit in Modern Warfare 2 (on Xbox 360) is allowing griefers to reset their opponents’ ranks back to 1 by awarding them a large amount of negative XP at the end of the round.

ME and my clan (SC) went into a game of HxC SnD, we found a group of tenth prestiges, we thought they were merely boosters, we were very wrong.
We knew something was wrong when they just waltzed out of their spawn in Highrise and let us kill them.
After we won that round, they said “AWW look at the level 1’s” and we noticed…
Killing them made us LOSE xp…. we won the match cause they ended it.
Match Results: -899999453xp….
What the heck…..
I was 2nd prestige rank 50, now im 2nd rank 1. everyone else in my group is reset as well.
to add to the weirdness:
all of my guns, camos, challenges,attachments, killstreaks, all still set as I had them in my Custom slots.
BUT when i went to customize, they were all locked as if I didnt have them.
So we decided to go play a match, I can rank up and everything, its as if I never got to 2nd. Rank 50.
People Say pics or it didnt happen. heres 2 videos of the other clan members (Including
myself) showing our loss:
SC ShadowTSI: [ Register or Signin to view external links. ]
SC KINGCOBRA: [ Register or Signin to view external links. ]
This Needs to be addressed, or else almot everyone will eventually have their stats reset.
The good news is that prestige ranks, titles, emblems, killstreak rewards and already configured custom classes remain untouched. The bad news is that all previously unlocked weapons and perks are not available to when editing or creating a new custom class.

No official statement from Infinity Ward has been made. Unfortunately, both PC and Console versions of Modern Warfare 2 have been plagued with exploits.