“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.
“The teams are actively assessing root cause of the source of the leak and have already implemented mitigations for future flights,” SpaceX stated on its website, but did not provide additional details about those mitigations.