I have shoal of 6 neon tetras along with some other fish in my 15gallon tank. Recently I've noticed that one of the neons has stopped shoaling with the others, and when I sit down and watch them I notice that 5 of them just chase him away when he tries to swim near them and try to nip him, it's as if they are all "ganging up" on him.

He's fine around the Danios, but always seems to be hiding in the darkest corner of the tank (behind some plants near the filter) or in the little cave thing I have in the tank.

He only comes out to eat, but doesn't eat much.
Today I noticed though that he swam around for a while for about 5 minutes after eating, with no problem with chasing or anything, before he went back into the corner of the tank.
Why have they suddenly turned on each other? Is there anything I can do? Or should I see if he'll swim around more everyday like what happened today?
Gwenz