The simple fact is, you never know what the reason might be in
online poker.
Perhaps they have an infant and it starts to cry and they have to suddenly leave to tend to their baby... perhaps they suddenly had a case of... well, I won't get graphic, but let's just say they had to run to the bathroom quickly... maybe they spilled hot coffee on themselves and ran to the freezer for ice cubes... maybe they suddenly had a heart attack, or maybe the police suddenly showed up pounding on the front door. LOL All these cases would result in a player using up their entire time bank in places that obviously don't require it (level one in a 5-hour-late-reg tournament). I've done it myself! I forget to click the sit-out button and just walk away from the computer, absent-mindedly, and I return to realize I have zero time bank left... ugh.
So its usually not a player who is thinking too much about their hand, that's what I'm trying to say. You just never know what the reason is, but poker is a game involving much patience, and this is just another spot where it doesn't benefit us to get mad or angry at these players. And its also why I'm rarely focused completely on my tables in the early going, I'm usually watching television off to the side or maybe even playing a video game along with my poker. But that's just my solution, everyone handles it differently, but my suggestion is, don't worry about it when this happens. This is why we are given time banks, just be thankful that when someone leaves the table, it usually only slows the game down for one turn, once their time bank is used up, they won't affect the game very much after that hand.