Why would you even sell your altcoins when in fact, the forking will not affect these altcoins? Bitcoin forking will only be within the BTC network, and altcoins will remain the same, unless people will really sell theirs. Once the altcoin holders sell their altcoins and invested the money to bitcoin, then the possibility will be that altcoin prices will drop and BTC will rise up (just like what's happening now with LTC). But then, there's nothing to worry about your altcoins with the coming of BTC forking.