This has happened to me (big winner turned loser, and then back again). I was the top winner in my regular weekly cash game, with a win rate that was almost absurd for the stakes, like 30+ BB per hour at a game with a short buy-in structure. (I believe my peak was somewhere around 33 BB per hour.)A +10BB/hour win rate is hard to suppress via bad luck. ( same thing with a -10BB/hour turning to wins ) Of course a winning player can get off his/her game and spew money. But it isn't due to bad luck alone, the causes are primarily bad play. Occasionally a wave of really good new players or really bad new players joining the game will shift the balance of power. But this should be glaringly obvious.
I won almost every week for like a year and a half, and it wasn't just a run of good luck. I was absolutely crushing the game, partly because I was more experienced than most of the players, and partly because the game often got into conditions that massively favored me. Specifically, we'd quickly go from a full ring with short stacks to a short-handed game with deep stacks, and we'd play short and deep pretty late into the night. My opponents did not adjust so well.
But then the game changed. A new skilled player or two got into the game (the audacity), and the host changed the buy-in structure in a way that emphasizes short-term luck. After that, the table tended to stay full all night instead of going short early, which is great for his game, but it really put a wrench in the works for me. The combination of the structure and the players was much harder for me to beat than it had been (and also more variable), and I spewed an embarrassing amount of my winnings back before I finally got back on track. One week, I was suddenly back on my game. Win rate's probably more like 10-15 BB as of the last time I played, and that's fine by me.
Anyway, my point is that it happens. My case is more technical in nature, but it just as easily could have been that I was going through a hard time at home or work, drinking too much, or whatever. Could be that any of these things are true in OP's case as well, though based on the little information I have, I'd guess the player in question is just a marine mammal on a heater.