Here's the only options as to why:
1. You're a booster who got caught.
2. You accidentally joined a modded game and got reset because of Treyarch not fixing shit.
3. You got the unfortunate side effect of some glitch.
As you can probably tell by number 2, that happened to me in the first Black Ops. Was prestige 3, level 49 and only 20,000 XP needed to prestige. I joined a match in progress that just started, got the first kill, got 50,000 XP and quit the game immediately. Still got fucked over and reset and then I metaphorically shit all over Black Ops 1.
At least check to see if you still have your titles, emblems, and leader board stats as I'm pretty sure if you use the reset token it wipes all of that.