It's the trading of ideas in a highly competitive industry. Ideas and themes of potential hit movies have been leaked, sold, obtained by agents and spies for decades and has itself been the subject or part subject of several Hollywood movies. There are probably examples from almost every year when two similar themed movies are released. The timing can be almost identical simply because the 'copy' wants to be released first to establish its originality in the minds of the audience and critics.
If the insider pulls off a clean job ie. doesn't get rumbled by the prod co. who've optioned or commissioned the first piece then the rival prod co. the agent sells it to has the time to make a decent picture by not rushing, especially if the insider passes on top info like projected finish date of filming and release date by distributor and copies of the screenplay drafts. It can make the agent richer than the writers, especially if it's a hit and the poor copied original movie screening weeks later then gets labelled as the copy or rip off. That, they say is 'Hollywood', or more generally now the whole film and TV industry. It can also happen when an unscrupulous agent has sold similar screenplays to two rival prod co.s. If they air exactly together then it can signal a compromise, someone smells a rat and the two broadcasters agree to nullify the other, to avoid legal action.