Actually bitcoin is like others coin so its price can be high and low. So if people see that the price of bitcoin is high so they can sell. They can save something for the future also. There is nothing as stupid decision.
Selling bitcoin to take profit is not a stupid decision. That is a wise decision to take profit as people invest money to get profit. Selling bitcoin can be taking profit or cutting loss or minimizing loss and later can buy again at lower price. That is one way we can take to control risk instead of holding for longer time.