Hi and Welcome,
People usually sell due to FUD (Fear Uncertainty and Doubt). If one does not believe in the intrinsic value of Bitcoin and has bought Bitcoin as a speculative tool just with the hopes of making profit, when the prices dip such people give into FUD and sell their coins to avoid total capital loss not knowing that it is normal for the seasoned investors in the cryptosphere.
I wouldn't! Dayum I did not sell my ZEC for so long but I finally gave up and sold it today.. because it was eating up my potential earnings so I did not want to stay invested.. That's another reason why people might sell... If they see that they can get a better return in holding another altcoin if the current investment does not seem to benefit you as per your set targets/goals.