Well, it might be one of a few things I would guess.

People might be selling alt coins to purchase Bitcoin.
Or, its perspective. Alt coins are valued vs Bitcoin. So, its not that the price of an alt coin is dropping by a lot, it's that by comparison, it didn't rise as much as Bitcoin. (it's the same way with current fiat money. Everything is more expensive... it's not... the currency has less worth.)
Maybe people are moving on to the next ICO and withdrawing from the current markets.

It's hard to say. Crazy place out there.