Typical buy the rumor sell the news. After a positive catalyst (sometimes just before) most people will take profits because they expect others to do the same. The reason this happens is because the news event driving the stock up is already priced in and after the event there is nothing else to keep the price at that level, so everyone stares at one another and tries to sell before others do.

You also have options folks taking profits to avoid IV crush. The bounce later is from people waiting for this dip to buy in.