alt Hacker NewsI watched the launch until shortly after the solid-rocket booster separation. Up through the booster separation, things looked pretty good. The boosters didn't have any obvious anomaly and nothing was said in the commentary ~ minute after SRB separation.
I tuned out at that point thinking they had gotten past the hard parts. Genuinely surprised to see this news in post.
Agreed--everything looked good. Booster engine cut-off was ~40 seconds late, though, and stage separation happened lower and at a slower speed than normal. I think at that point, something happened with the Centaur/payload. Either it couldn't handle the aerodynamic forces or maybe it deviated from the corridor and FTS fired.
The ULA animation also showed some strange rotation at stage sep and maybe even a faring bump the Centaur (knocking it off a bit).
Maybe the underperformance of the booster left the upper stage too low and atmospheric drag caused problems at stage separation.