Bitcoin price will drop with time due to transfer time, which now can be up to 3 or 4 days, with average fee 0.0002 BTC. Therefore people will switch to altcoins, which are much more fast, and Bitcoin price will drop, altcoin price will go up. Bitcoin Cash transfer for example take 20 minutes, Doge - 10 minutes. So this is time to sell your Bitcoins !!!
Another reason will be - large miner farms mine Bitcoins, but Bitcoin user group is the same and not expanding, so Bitcoins just loose its value due to large scale mining too.
I think it's the opposite of what you're saying. Right now, the best thing to do is to buy bitcoin. We all know that its price drops because of every 4 years halving . Not to mention that it's price varies ovetime. On the otherhand, you cannot predict the price of the bitcoin. Some chooses to invest in bitcoin rather than altcoin even if the price drops because they believe that bitcoin is going to pump up again after the halving.