I don't get what you try to say, because bitcoin always stays at the top.
Yeah, it stays at the top of the coinmarket cap list, but that doesn't mean bitcoin doesn't crash--which it's in the process of doing as I write this post. And since altcoins like ETH are priced in terms of bitcoin, they're all loosing fiat value if not bitcoin value. So the ranking stays the same, basically.
And who the hell knows why mini-crashes like this happen. I'm disappointed to see bitcoin below $7k again, but it was only a drop of about $400 (yeah,
only). There have been much worse declines before, so my eyes aren't exactly popping out of my head. Yet. Let's hope next year is going to be a better one. 2019 started off with bitcoin less than half of where it's at now. It's good to remember that.