We got some additional info on the cause (from another site):
“During the coast phase of this Starlink mission, a small liquid oxygen leak developed, which ultimately drove higher than expected vehicle body rates,” SpaceX stated. The company instead passivated the upper stage, removing sources of energy that could cause the stage to break up. That is typically done by venting propellant tanks and discharging batteries.
That is the 3rd failure of a Falcon 9 upper stage in about 6 months. In July 2024, a Falcon 9 launch of Starlink satellites suffered a liquid oxygen leak that prevent the upper stage from performing a circularization burn. And in September, the upper stage for the ISS Crew 9 mission had an “off-nominal deorbit burn” that caused the upper stage to reenter outside the designated zone in the South Pacific Ocean. This latest one was a completely uncontrolled reentry that landed in a populated part of Europe.
Unlike the first stages of Falcon 9s, which have flown as many as 27 missions per vehicle, the second stages are not reusable. So, these problems with second stages are not due to wear or use, they have to be quality control issues. That may, or may not, be related to the rapid pace of launches the SpaceX is performing.