Why would Ethereum cause a drop in BTC?
BTC can drop Ether's price, because even today most ethers are bought with BTC, which is the coin to trade other coins with.
But I don't think Ethereum has that much power...
i agree with that also. ethereum is a known platform but it doesn't mean that it has the power to change the market, bitcoin in particular. most of the altcoins' price follows the trend of bitcoin. it's not the other way around. there are many underlying factors also but definitely, eth alone didn't cause the price drop of bitcoin.