Of course they did, bcs they see that the price of BTC is more stable and every day is more higher than the previous day. Nobody won't to risk his money, and they know that btc is more credible than the other alts.
Bitcoin has become more stable than it used to be without a doubt and with more users it is difficult to manipulate the price like the did in the past and so is the reason we are seeing a stable price,but still if you are seeing a good alt with a good entry point it is always good to make some good money with it as you can see a pump almost everyday in the alt coin market.
The price of Bitcoin I think never be stable, we can see that in last week or last month
that the price will be down and rising fast, it's because the price of Bitcoin always be volatile.
IMO The people sellings altcoin and go back in Bitcoin just for long term investment.