time lag
You're most likely seeing lag and if the game is counted as lost then you ran out of time on the server before you could make a move.
It could be due to high lag between you and the server. Check out the following for some things you can do to minimize issues.
https://support.chess.com/article/4720-online-chess-performance-optimizations
https://support.chess.com/article/213-how-do-i-fix-my-disconnect-lag-issues
If you need help, please contact our Help and Support team.