It's probably too early for it to happen. We could have seen some drop in price for altcoins for last couple of days, but seems like they are going back on track because as I said, it is too early to blame BTC fork for it.
But definetly couple of days before BTC fork, a lot of people will dump their altcoins to get more Bitcoin Gold or whatever it will be. The same thing that hapenned on August 1. with Bitcoin Cash.