On the lockup expiration data (usually 180 days after the IPO) shares can be sold by insiders without an SEC filing.
Sometimes the extra stock supply causes a decline in the stock.
Twitter is an example. Their lockup end date was May 6, 2014. May 5 the stock closed at $38.72. May 6 it closed at $31.80, down 18%.
A word of caution. What happened to Twitter is an example, not necessarily a general rule.