The tetras may be nipping each other trying to assert their dominance. Mine have chases each other around but have never ripped the tails. They've nipped at each other, but not that bad. The bumblebee goby may be the culprit. They are agressive fish at times and I have seen them smash into snails on the side of a tank trying to dislodge them or possibly get a bite out of them. They are quite interesting fish. Hopefully the tail nipping stops when they know whose boss.