He might not be in the mood on that occasion. I'd be shocked if no one on this site has never been hissed at. Most have probably been bitten. They are very docile, but sometimes don't want to be disturbed as they may be going into shed (not malting), you may have shocked it or scared it, could be any number of reasons. Nothing to worry about, just got to remember it is a non-domestic animal, and will still have the mind of a wild animal.