We don't know all the things they did to the Booster, but among them were deliberately not igniting some engines as well as taking a more aggressive angle-of-attack on descent (the rocket is a fairly effective lifting body, as it turns out!).
There may be more things, but between those two I think the latter was a bigger problem. It would have gotten hotter and more physically stressed. And then weakened to the point to where re-igniting the engines caused it to fail.
They also used a new hot-staging maneuver, where the gases were directed out one side so that it flipped more rapidly in the other direction. It was a really fast flip! A rocket the size of a small skyscraper turning 90 degrees in just a few seconds. That could have jarred something loose, too.
Hopefully we find out in the post-mortem. SpaceX doesn't typically give the public as much detail as we'd like, but they're pretty good at sharing the high-level reasons why something failed.
There may be more things, but between those two I think the latter was a bigger problem. It would have gotten hotter and more physically stressed. And then weakened to the point to where re-igniting the engines caused it to fail.
They also used a new hot-staging maneuver, where the gases were directed out one side so that it flipped more rapidly in the other direction. It was a really fast flip! A rocket the size of a small skyscraper turning 90 degrees in just a few seconds. That could have jarred something loose, too.
Hopefully we find out in the post-mortem. SpaceX doesn't typically give the public as much detail as we'd like, but they're pretty good at sharing the high-level reasons why something failed.