...we don't know.
In fact, Space X are so stumped they're asking not only NASA and other organizations for help but the general public - they're asking anyone with video or audio recordings of the incident to send them copies.
The rocket was being fueled at the time and the company has said there was no heat source that could possibly have started the fire.
What they do know was that there was a bang and then the "explosion" happened a few seconds later.
Most likely there was some structural failure - but nobody can rule out something hitting the rocket.
It probably wasn't aliens, but it's not impossible that it was a UFO.