Unless a particular alt coin market is strong enough, this happens due to one or the other of the following reasons:
1. To balance out the value in Dollars. Basically, to prevent Arbitrage. (Please google for detailed explanation)
2. Demand and supply. People trade bitcoins
for altcoins and vice versa.
Also, this has been a repetivtive pattern for all the past events when BTC has climbed up rapidly. So, as a practice now, traders tend to move out of trades to prevent losses, in turn preventing themselves from making any profits. A vicious circle.
But, If you would observe closely, all the markets were quite stable during this last/ current elevation. I don't know for sure, but maybe the incoming volume forecast because of the CME futures, movement of BTC will not have much effect on that of the alts. Infact, itishould should keep the price free from sudden fluctuations.
That's totally my opinion though after spending a while in this space. Let me know yours. :)