Sick clown?

Hello,

I came home from work on my lunch break yesterday and thought that I was
missing a clown fish. After putting a bit of dry seaweed in, he came out
from hiding in a far corner under some pieces of live rock. For my
clowns, this is VERY unusual behavior. They usually don't even go down
to the rock level let alone hide stationary under it. When he tried
getting some food, the other two attacked him and he went back into hiding.

These are all Occelaris clowns, tank bred. Normally very gentle and have
been together in an extremely close-knit group for months without
incident. I'm at work right now, otherwise I could look at my logs and
tell you the exact date they were added.

Anyway, I moved him to my pseudo-quarantine tank that basically just
houses my small pyjama cardinal and two small cleaner snails to keep it
going. After a lot of observation of him while not being attacked, I can
find no spots or anything that might indicate disease, he swims
normally, and just seems perfectly normal. He does have a nipped side
fin since being attacked that wasn't there previously. On an amusing
note, because of that he looks even more like Nemo. Same side even.

So far I'm keeping him isolated and keeping a watch on any new
developments and will probably keep him in there until his fin regrows
at the very least. Any ideas as to what would have caused the other two
to suddenly attack?

Thanks,

Brandonb