I have ran accross this myself before. Just the other day, I had a clear win My clock was at 7 minutes, my opponents clock was at 8 minutes. My opponent said "You got me, you win" in the chat window. I was up a whole rook in a great position.
I went in the other room to get a pop, and came back 2 minutes later still no resign... so I just sat there and watched the clock... when my opponents clock hit 1 minute, he decided to make a move.
I replied in less then 5 seconds, and immediately he resigned.
What was he doing? It was simply a tactic of trying to swindle me out of a win by getting me to run out of time by going afk.
happened to me today on a 30 minute game... the guy actually told me after two moves "bye bye". I said "Are you done playing"? The guy said "yes"...but the clock kept running. I politely asked him to resign and he sent me a smile and said "i'm not doing that"...he refused to resign and refused to accept my draw...and I had to let the clock run out before I could start another game. Definately frustrating to deal with.