Buy the rumor, sell the news (Don't get caught bag holding)
Huge government contracts can often yield positive results for companies (AJRD)
SPACS or Special Acquisition Companies are basically "blank check" investors that attempt to partner with another company to take public (able to have their company in the stock market) and help to raise funds for that company
People buy the rumors of upcoming mergers
When a merger looks complete or right before it is announced, people sell off the hyped stock which causes a decrease in price
There are no guarantees to your ownership as shares could be purchased back at a cost of $10/per share plus interest (though many companies often allow current shareholders of the SPAC to retain their sharess). Shares purchased over that amount would lose out on the value.
Announcements about the company (e.g earnings, dividends, setbacks, legal issues)