Opponents clock increases its time when down at less than one minute.

But in looking at some of your games from today you were playing with increments which adds time after each move.

Sorry I did not make it clear this incident happened in the end game when opponents time left was less than 1 minute.

What likely happened is that you had a brief disconnect and the server may not have received your move.
The server is the keeper of the accurate time. With the lag compensation algorithms the times on the clients may not be accurate. That is why you may occasionally see clocks adjusted in games, even without increments.
Perhaps I am missing something obvious to other players. But it certainly is disconcerting when yor opponents time increases when he makes a move. In my case it gradually increased by 40 seconds until I gave up and just abandoned game.