Not all the time, but occasionally when the price of Bitcoin crashes down, the altcoin market also drops most of its value. The reason behind that in my opinion is that most of those coins are traded against Bitcoin and that is why when the price drops, they drop accordingly. I can be wrong though.
There are also some people having a lot of coins and manipulating the markets every now and then to get any amount of profit out of the pump and dump.
Yes you were right some of them are manipulating the outcome of the price and we called them 'whales' those people had a bunch of money to manipulate everything in order for them to gain a certain profit by just pumping and dumping into almost everything. Yet it is a good deal to buy Bitcoin though since it has started to go down a bit but i wonder when will it last.