Selling when the market is going down signify two things, first is, "panic selling" simply because of fear; secondly, inexperience as a trader/holder on a news or speculations that a big crash or market dump may occur.
And lastly, lack of confidence on a particular coin/token that your are holding, probably because such project coin/token has no usefulness or usercase. Trade or hold your coin/token with courage, anything can happen.