People make mistakes when they see profits, and they sell everything. Take a look at its potential for growth. Try to keep Altcoin potentially in the long run as this is necessary. You should only sell them when the market goes down, after a fall you can buy at a lower price.
Some coins make incredible price increase after hitting new big volume exchanges. In some actions like this, we cannot predict if we should hold or sell those coins. Like this person, some people missed big opportunities.