It's seems counter-intuitive but it makes sense when you think of it like this:

The less USD in the BTC market, the less USD that can be poured into alts. So both BTC and alts go down in price compared to USD.
To compound this effect, when there are bear markets, most investors will put their money into something perceived as more stable. So people will want to lock in the best BTC price they can find so that when BTC/USD recovers, their overall financial hit won't be as bad. Usually when there are bear markets, a lot of investors are significantly risk-adverse. The funny thing is that it also follows basic human psychology. When things are looking bad, most people want to find the thing that will be the least detrimental.

A lot of the time, this has the effect of stemming the bleeding in the BTC markets, at the cost of significant impact on alt markets.