how did this happen?

I lost a game on time when my opponent only had a king and a knight.  How is that possible?

You lost a game on time with a knight... why don't you show us your game? perhaps it was your turn

If you run out of time and it's still possible (by any series of legal moves) for your opponent to mate you, you lose. Them's the rules.

Your opponent had a pawn, which can turn into a queen, which can deliver checkmate.

my opponent had no pawns only a knight and a king.  So isn't it impossible to mate with only knight and king?  Yes it was my move and I ran out of time but I thought the system granted a draw for insufficient material for a win?

Hmm, I was looking at the wrong game it seems. Did you have anything aside from a king which would make a help mate possible? This system is never wrong in my experience. 

The problem was that you had a pawn.  That pawn could promote to a Rook, then the following position could occur.  Thus, mate is possible, and a loss is the correct result.

Good explanation.

the problem is helpmates like this could occur