It is logical movement. When people have raised some money, due to the BTC price increase, they are selling their BTC and buy some alts, that have not pumped yet, with the hope, to get more profits. It does not work every time, but it is a great strategy.
Yes youre right but I dont usually do that. When the price of Bitcoin pumps and I make profit and withdraw my profit, I will immediately look for some altcoins to invest little of my money with hope that that altcoin will have a price increase at any time soon. But I dont sell off all my Bitcoin, I still leave some of them cause I cant predict what might happen next.