Altcoin will start increasing when bitcoin is stable at some price for some period of time, Now people are investing in bitcoins due to which demand of bitcoin increase and demand of altcoin decrease so when bitcoin reach their highest price then people start selling bitcoin and buying altcoin.
I do not think that it is important that when bitcoin price start decreasing. other altcoin also start decreasing. It is a fact that when bitcoin price become a little stable then people try to convert their bitcoin into other altcoins where they have good chances to increase. but when they realize that bitcoin price again going to increase then they again convert their money in to bitcoin.