I agree it is an interesting phenomenon. I've also noticed other interesting things, like the ETH/USD value dropping sooner than the ETH/BTC value during dips.

With the current bottoms well established I feel like nobody holding alts is willing to sell for these BTC prices. If they were they would have done so long ago.. so this is why the BTC value seems more stubborn than the USD value. Those who are in it for fiat see the price swings and cash in and out, but those who entered through BTC are holding firm. I think this is going to cause alts to go up