It happens.
They may have been fighting, they may not have been. I've not heard of it in cresties, but gargoyles have a tendency to eat the tails of cagemates.
I find the 'flightly' ones tend to be more likely to lose their tails just out of fear or spazzing too much.
The last time I had one lose a tail was during a half hour drive to move her to my new place. Just enough discomfort during a short car drive to make her uneasy enough to drop the tail.
They may not look as nice, but frogbutts are still cute. Keep an eye on your pair to watch for fighting.