Why?

http://ru.lichess.org/J8010JqdMUem I was winning this game i made move b5-b6 and my oppenent began to takeback and even asked for draw. Sure I refused and i lost by time. Can someone explain why? I'm very angry.

Two tackbacks were accepted.
If you accept a tackback it's like you didn't done your last move. So the passed time goes to your clock.
If you accept a tackback while beeing low on time you may lose on time.