I sometimes have the same thing happened with one of mine. Is your fish coming out to eat? Have you done any thing new to your tank lately? Next time it comes out take a good look at the general health of it. I would monitor the behaviour the next couple of days
I know I'm coming in late, but I've got the same problem. I inherited one neon and got 6 more smaller one's. The bigger one hides in a log and chases other fish out. He always kept to himself, but lately he and all my other 6 have been chasing each other and fin nipping. Water parameters are good. Temp at 77, pH a little low at 6.0. My tank is about one month old. It happened suddenly. .