The reason for this is quite simple.
Currently people are trading their altcoins for Bitcoin to join the speeding bitcoin train and make some profit. However the train will stop eventually. After Bitcoin stops growing so hard and becomes stable or drops a little in price the people will trade bitcoins again for altcoins.
I don't even have a concrete idea about this, but I think one reason is that there's a lot of altcoin nowadays and since there's a lot of choice where to invest to, people are rushing whenever a new potential coin appears so, they would trade their old altcoin to invest to a new one. That would affect the price of other altcoin. I'm not quite sure, but I think it some how makes sense right?
For me the scenario that you observed was due to the higher price of bitcoin in the market exchange updates from the site now, which really affected most of the altcoins holder to dump their assets. So people's investments is now very agressive and wanted to move for a new coin to continue for another journey of altcoins to be promoted. With those more promising coins to be added in the market soon, I am pretty sure that would br dumped immediately with it's added at trading exchange site, that's why big opportinity comes along.