There are a few reasons, some examples (not limited to the below of course):
People will buy coins before forks, in the hope of claiming both coins and getting a "dividend" even if they don't believe the fork coin has value, they can simply sell at any price to get "free" money. Possibly these people will then sell the coin they bought for bitcoin anyway, benefiting from the rise as other people bought in, and the sale of the fork.
Another possibility, if you're a BTC maximalist (and a "Whale"), is driving alt coins up, after the coin has pumped, you can sell back for BTC, increasing your BTC holdings. Buying randomcoin and a few cents, driving the price up, or waiting for an announcement etc, will mean, again, you can safely move back to BTC increasing your holdings. There's some circumstantial evidence of this happening.
People realising "gains" from alts, moving to BTC, people tell you to "diversify", but that's not always the best long term strategy, especially if your altcoins have experienced a gain, moving profits back to BTC (in lieu of FIAT) makes sense.
Dont forget the power of FOMO (Fear Of Missing Out), people will buy anything if the price is rising!