Some <insert bad word adjective> players decide to pull and then /leave the Duty to kill the whole party thus wiping them all (ie. Ifrit hard).
There's also some kinds of the same players, who decide to instantly leave the duty because there's no (unnecessary) mage in the party to LB3, thus screwing another 7 players again.
The first can obviously be fixed by disabling /leave during combat.
The second is about punishing more players who leave. I think 15mins for leaving is good enough, however, leaving in a reiterated way should increasingly punish the player more (for example, the 2nd time in a row you leave, 1h penalty).