Question about time limit in arena.

Sometimes when i'm playing arena, when i'm in a battle and I have to do other things and put the smartphone in standbye, when i back the screen sometimes the game is still in a battle, sometimes appears a prompt showing 1:30 left, and sometimes i back with a defeat screen, so how does it works the time limit in arena? Its only happen when we have the screen on and without doing moves? Because in auto-play the game never finish because times, when we have loop battles. I would like to know why this 1:30 left appears when i left the game a few minutes in a battle.