two year old starting to hiss

I have 5 cats. Started with two, but got to the boy kitty a day to late.
Anyway, Tigercat, the father, has just recently started to hiss at all the
other cats when they get within about 5 feet from him. He has been with these
cats since they were born and (to my knowledge) has never done this before. It
started about 3 days ago. Nothing big has happened (recent move or trip to vet
or anything).
They are indoor cats. Any thoughts?


-John
*You are nothing until you have flown a Douglas, Lockheed, Grumman or North
American*