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

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 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