in case of ETF approval we will first see a bitcoin price surge. since this was a long awaited rise a lot of people will move their money to invest in bitcoin. that money can partly come from their altcoin investments so as they pull their money out we may see a big drop in altcoin prices and since they have been ready for such drops the down-slide can be severe and huge.

considering in the past during bitcoin price surges like what i described altcoins usually get dumped i think the "drop" speculation is a likely one.