It is possible if you can make a big community with a big money in their hands on bitcoin. There are some ways that can make this things happen, such as :
1. Pumping another coin by shifting their money to that coins.
2. Making a fake news so people panic and sell their bitcoin.
3. Selling some of their bitcoin to real fiat currency.
That is what i can think of the way to manipulate bitcoin prices. Big people really can manipulate the prices, since they have a lot of money on bitcoin and, sometimes big people are friends in real life, so they can arrange and make this happens. But i hope they dont make it often
For small investor like us,it is a scary things to face, but i hope we can make it through wtihout panicking and making a bad decision over it.