actually there always is a reason for any rise of all it is just that we can't find that reason in half the times when the rise of fall is happening and usually we figure it out a while after.
Imagine 3 whales with 1000 btc each decide to sell bitcoin and by chance they do so at the same time. Of course the price will tank and people will wonder why, bots will follow and so on.
not every time. it will depend on when and how they sell large amounts. for example if they sell it all in one exchange, nothing will change. if they spread it among exchanges then the amount may not be enough to make a difference but there is a better change.
and most importantly is the timing. for example when people weren't sure about the direction and many were feeding them FUD about how price is going to $1000, such sells crashed the market about 50% from $6000 but if the same thing happened today it won't even cause a 5% drop let alone 50% because the market is on the move and such sells would be against the direction and will be eaten by the market like a piece of cake.
Of course not every time but considering how 80% of the volume is fake its safe to assume that someone who is rich enough could move the market even if he doesnt want to. Clearly sometimes the price moves according to news or upcoming events but sometimes it also moves simply because of TA indicators or coincidences.