There are times when theme parks close much earlier than expected. For example, you might see a scenario where a park closes at 10:00 p.m. one day and then 4:30 p.m. the day after!
Oddly early closures are generally due to some sort of special event occurring in the park.
Sometimes these are public, hard-ticketed events like Mickey's Not-So-Scary Halloween Party. Such events will generally be noted on our Crowd Calendar and on the Disney or Universal websites, and the public can purchase tickets (subject to availability) to enjoy such events.
Other events are private. For example, a large convention might rent out a theme park for an evening, or Disney/Universal might host a private event for its employees or some other group. In such cases, the public has no access to the park during the private party. These private events are not necessarily well-publicized, so - even if you're curious - it may not be easy to find out what is motivating an unusually early closure.
Finally, note that even "official" park hours (including ones affected by special events) are subject to change, so we encourage you to monitor park hours as your trip approaches.