I suspect there are a number of reasons but the most basic one I can think of i that alt coins are priced in BTC, if BTC rises and their price stays static then technically they are worth less BTC.

E.g.
BTC = 100 & ALT = 10 (10% of a BTC)

BTC has a good day and increases in value because lots of people are buying and trading with it increases to 120

Now our ALT is still worth 10 (Now only 8.33% of a BTC)

So if you look at the numbers the percentage worth of a BTC is down. On the price charts it will look as if it has suffered a 1.7% drop but technically it is still worth what it was before just that BTC is a moving target and if it is priced against that it can look as if it moved up and down when it doesnt.

Lol, I hope that has helped. There are a lot of other factors, one that comes to mind is that people sell alts when they see BTC rise lowering the ALT price and then buy back in when the ALT has dropped. ITs a complicated thing!