It could be that the tips of the tail have been damaged in some way. My male has done something similar with his tail on a few occasions, and I suspect his might be down him getting it 'sucked' into the filter intake. I tend to keep a close eye for any indication of infection...if you see any cotton wool like growth it could be fungus that may well need treating with salt baths..and make sure you are not lax with tank maintenance. In Jeffreys case it tends to heal up nicely before he does the same thing again!
