im sure, if some people sell their coin, it doesnt mean that they are not believe on bitcoin. i think, there are many reason why people sell their coin. maybe they pay debt, or they out of stock money and just depend on bitcoin, so must be sell their coin. and im sure none try to sell coin because of afraid about market cant recover, because someday it will recover again
there are several variations of coins that have stable prices, there is another reason than you mentioned is that they choose to play safe while not too monotonous to survive with bitcoin, other assumptions are also related to trading various altcoins and they must sell their bitcoin assets to prospects profit. But I'm sure they also know when it's time to go back to Bitcoin.