We had a similar leak in our shower drain. It was due to the pipes coming apart downstream of the elbow, but no crack - I just reconnected them and no problem since.
I assume it's due to the rigidity from the inlet to the drain all the way to the main pipe.
If it happens again, I'm considering a flexible piece downstream of the elbow to allow the two sections to float independent of each other.
I'm pretty sure I know the bump I hit that jarred it loose
. The lack of support around the drain seems to allow enough flex to cause problems there if anyone steps on/near the drain.