.... Went to check another tab (without closing the Chess.com tab) whilst I was waiting, and came back to find that I was no longer in the tournament. I didn't hit withdraw, didn't leave the site other than to refresh the page, and when I did I was still in the tournament - what happened?
Apparently the next round started while you were in the other tab and you didn't get back before the game was considered abandoned. I don't know the exact amount of time but if you don't make your first move in that time limit, which isn't the time control, then you lose by abandonment and get automatically withdrawn from the tourney.
I'm assuming it was this game: http://www.chess.com/livechess/game?id=1223404964
I just entered a 3|0 blitz tournament. Lost my first round (nothing unusual there) but noticed I was having connection issues. Hit refresh to see if that would fix the issue (it's worked before), got my connection back to green, and I popped back in on the same game loss screen (including the Panic_Puppet lost on time message). Still part of the tournament, still 5 or 6 games in progress. Went to check another tab (without closing the Chess.com tab) whilst I was waiting, and came back to find that I was no longer in the tournament. I didn't hit withdraw, didn't leave the site other than to refresh the page, and when I did I was still in the tournament - what happened?