If you didn't see The Purge, which still includes me, it is a thriller about when crime is legalized for one night out of the year to ensure peace for the other 364 days. A family is put in danger after they rescue an injured citizen during the purge. I haven't seen it yet for the sheer fact that it took forever to go on Netflix and I bumped movies ahead of it.
The Purge did seem to be a success, though. Universal Studios Hollywood even picked it up and create a "scare zone" during Halloween Horror Nights. A sequel isn't necessarily surprising.
Here is the trailer: