That link doesn't point to a game against the computer. But the usual reason for unexpected draws is that the clock ran down on your turn, when the opponent didn't have enough pieces to give mate and get declared the winner. Without enough pieces he just gets a draw.
Issue with finishing game vs computer

The link points to the game I had won via my opponent's resignation. I was using the "finish game versus computer" feature available to paid chess.com accounts.
There is no clock in the "finish game versus computer" feature.
I was trying to finish this game vs computer:
http://www.chess.com/livechess/game.html?id=284374510
(OK, I know it's an obvious win, I just wanted to get to the checkmate).
In the "play position against computer" window, the moves went 1. --- Ke7 2. Qa6 Kd7 3. Qc6+ Kd8 4. Qd6+ Ke8, and at that point the system declared game over, draw.I don't see why it was declared a draw.
I had planned Qc7 as my next move, but the game over message came up before I could make the move. I tried the game multiple times, and every time after 4 moves it was declared a draw even if the black king had moves.