OK, this only happened to me twice so it seems to be not really common:
Playing as allies, we captured the Old City, repaired the water pump and approx 4 ppl shortly thereafter dived through the flooded tunnels. Now, everything went smooth but when I reemerged to catch some air shortly before the tunnel ended I was suddenly ‘pushed backwards’ ! It was impossible to advance any further, if I didn’t pressed my ‘move forward’ key I was moved against a tunnel wall! From the 4 man team two of us had this problem , so I’m sure it wasn’t just my machine 
I tried for approx one minute to advance somehow but nothing worked and so I had to /kill myself and respawn.

