Has this happened to anyone else? What might it mean? Advice?
It could mean a variety of things. Could be you did well in most interviews, but not too well on one. You're probably on the border and they see potential in you so they want more interviews to see if you're up to the standard. Just more data points to make a decision. I've had it hapen to me. Keep LCing, provided this is for SWE.
The Google hiring process is just ridiculous.
How did it go?
It happens but is not common. Your HC was split and needs additional info. Ask your recruiter which competency they need more info on to help you prepare