Ofcourse sometimes really EVERYTHING goes down when BTC goes down, but i think it shows that people want to make use of the current cheap Bitcoin prices, so they sell altcoins that are on a high price now to have maximum profit and buy BTC because that's so cheap now.
It's the day-traders that cause this high correlation.
Yes, now this happens from time to time, but it was a rule almost 1 month ago (all alt-coins going their way, totally independent of bitcoin). But, now, most days, if you check all graphs of the main 200 coins, 95% fall when bitcoin fall, and go up when bitcoins go up. It could be concebible for the best 10 alt-coins (because maybe big groups are doing some kind of hedging), but if you take a look at the first 50 coins, most are related with the movements of bitcoin. And it was not like this until 1 month ago or so.
Bitcoin has been the basis for most alt-coins but most alt-coins movements were totally independent of bitcoin until 1 month ago or so. One could go up 34% while bitcoin was +4%, and other +21%, other -32%, and so on. Now, most are up when bitcoin is up, and downs when btc is down. It did not happen some months ago.