The purpose of HSX is to get players who have limited funds to invest in movie projects to help estimate how much box office each movie will do.
Good news (casting, trailers, posters, release dates, good reviews, etc) cause players to get more excited about a project so they invest and the price goes up.
Bad news (director/actors dropping out of a project, bad looking trailer, release date changes, bad reviews, etc) cause players to get less excited about a project so dump their investments (or invest short) and the price goes down.
In the case of The Inhumans, the project was announced years and years ago before Marvel started making their own movies and it never got off the ground so players were not interested and the price was very low. Then Marvel did announce that they would be making The Inhumans movie and set a release date. Players got excited and invested and the price went way up. Then Marvel got the rights back to Spider-Man and started switching the release dates of their already announced movies to fit a Spider-Man movie into the schedule. Marvel ended up taking The Inhumans off of the release calendar (it currently does not have a release date). Since there is no release date and since we now do not know when (or if) they will be making that movie, players dumped their shares (and some shorted) and the price went down.
Look on The Inhumans moviestock page. The second tagged news item:
BO: Disney's INHUMANS is now without a current release date (INHUM) Oleg Max on Apr 22, 16:55
You can also look at the graph on the right side of the moviestock page and see how the moviestock has been moving lately... it looks like The Inhumans has been dropping in price for some time now...
Read the news posted on the HSX Movie Board. Good news usually makes the price rise. Bad news usually makes the price drop. Check the daily change for each stock in your port and overall gain/loss. If you see red, then go to the moviestock page and see if there was any recent news posted.