[T]he Starlink team identified a common issue in this small population of satellites that could increase the probability of failure in the future. The satellites will follow a safe, circular, and controlled lowering operation that should take approximately six months for most of the vehicles. 

It’s a small fraction of the 5,828 Starlink sats in orbit, and SpaceX says it’s deorbited 406 sats previously. Unusual to decommission so many at once, though.