Injured zebra danio attacked!?

:help: After changing water today, I noticed that one of our five zebra danios was hiding. He looked awful -- black and thin. Then the others started atacking him. It was violent & relentless. I removed him. He looks better now -- back to normal color, but still more red around gills than normal.

I guess he got hit by gravel? But why would the others attack? Should I put him back or wait?

I'm going away for several days -- so I won't be able to observe. :help:

Thanks.