[Someone]

How(?) to reproduce the problem:

1. End a Battle
2. If you still have the turn timer icon, start another battle while it's still there.
3. The old turn timer will continue to countdown as normal, and end your turn if it ends during it.

Notes: And if you manage to finish your turn before that, the newly applied turn will count down as well and start the cycle all over again.

Workaround:
Don't start a new battle until the turn timer icon has timed out

I find this happens often while grinding in dungeons or the arena, since people have little to no time in between each fight.