Why does the southbound R train sometimes run express from Atlantic Av–Barclays Center to 36 St?

I've noticed that some Bay Ridge-bound R trains occasionally skip the local stops between Atlantic Av–Barclays Center and 36 St (Union St, 4 Av–9 St, Prospect Av, and 25 St) and run express instead.

Is this usually due to scheduled track work, dispatching to recover from delays, congestion around DeKalb/4th Avenue, or some other operational reason?

I'm curious what factors lead the MTA to have an R train bypass those local stations and whether there's a standard practice behind it. This happened during evening rush hour.