The one time it happened to me, it was for a flight on June 15 and it cleared June 11 at 9pm PDT. I was nominated by an E50K.

I imagine that either someone cancelled an R or J ticket, such that they opened up R space.

However, I can also see that on certain routes, they might typically know that they never sell more than 8 J seats after T-7. So at T-7, if there are 10 available, they set R2. But they also know that at T-3, they never sell more than 4 J seats, so if it's J6 at this point, they set R2 again.