I recently interviewed at very hot container startup. I had 1 phone screen, 6 on-sites, and 2 follow up phone calls. I did well in the phone screen to be invited onsite. Apparently, I did great on the 6 interviews onsite and then they wanted to have a follow up phone call with a manager (30 mins). That was positive too. But, the last phone call was set up with the SVP who determined that I was not a good "culture fit…
Maybe someone with exactly your skillset and experience was hired, hated it, and quit after 6 months. Maybe one of the other people you interviewed with mentioned an offhand comment you made and it soured the SVP's expectation. Maybe they just got served divorce papers and were shitting on everyone that day?
It's so random that I don't even try to second-guess or deconstruct the non-technical side anymore. It happens or it doesn't.