It is because of your internet. When you get disconnected on your move, and you play, it sometimes shows it like your opponents time is running out but its actually not. To fix this, get better internet or (I wouldn't recommend this) reload the page.
It's not a local problem. The problem is in Chess.com !
it is NOT a problem of internet connection, it's clearly a bug from when the servers are probably overloaded. We all shared the same experience: it looks like it's the opponent's move, the clock counts on the other player, and then, without any warning we loose for our abandonment (!!!) which never happened