Let me explain with some few illustration, in the past years, Bitcoin has risen from a price of $4k to highs of $60k+ meaning Bitcoin rise between 13-15x depending on which lows and highs you choose. Now when it come to altcoins, we have many coins that have done 100s of x which means people are most likely going to pull out more money form this project as they anticipate other Investors doing the same.
Also history doesn't favor the altcoins which is another reason investors would want to secure their gains as a result the market crashes more. If yo yo take a trip down memory lane, the altcoins market get the most corrections as coins correct 70-90% during the bear and just few survive this crash mainly because new and more attractive duplicate projects are created and they take the attention from the inferior/old ones also new trend takes over the altcoin market and the hype on the old trend dies down.