The video angle published by the BBC is better, it appears to show one side of the rocket disintegrating and sliding down non-explosively before the large explosion really kicked in. Would hate for this all to be described by a few missing bolts
https://www.bbc.co.uk/news/videos/cvgz0pdg32mo
edit: the failure appears to start at the bottom, this seems to have damaged the structure enough to cause the sliding to start, then the huge fireball seems to begin with a small flash closer to the top of the rocket
That clip is fairly low-res (at least for me). Here's the higher-quality source from Spaceflight Now: https://www.youtube.com/watch?v=1O90WZJALYc
the clip is wild. There's definitely anomalies originating from the bottom.
Then the spacecraft structure starts falling and tilting.
Then the explosion ripples up through the core and pops out the top.
Then the massive explosion occurs due to the complete loss of fuel integrity.
It looks like the bottom fell out, and then caused the pressure to ripple up through the structure causing complete structural failure. Wild.
The fish loved it: 0:29
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!