What you're seeing is not a 'side' of the rocket sliding down, it's the rocket itself. The other part on the right is the erector stand it was mounted to. Looks like the bottom of the rocket blows out first and begins to collapse. The rocket begins to slide vertically before it all becomes one large fireball. The erector stand didn't survive the explosion either in the end.
That makes a lot more sense, and sibling comment's higher res clip makes it a lot clearer. I knew I was posting a crappy analysis in the hopes someone who understands this stuff would post something more interesting, there is a dearth of technical speculation from googling around
No worries, it's going to be fascinating to hear what the investigation uncovers. Did the sensor data stream send back anything useful in the milliseconds before those sensors ceased to exist? I would assume there were no warning signs or they would have done a stand down on the test itself. So many questions!