There are only 2 reasons i can come up with:
1. Maintenance move to CI
2. 7 and 6 train R62A swap
Heres a way on how it works (although it may not be the official mta way):
7 train leaves corona yard and goes to qns boro plaza on the QUEENS bound track
Takes the diamond crossover just east of the qns boro plaza platform and heads into bmt 60 st. tunnels
Switches to the jamaica bound R track to QNS plaza. Gets onto the express tracks at qns plaza and heads west on the manhattan track. I am not sure why the train was backing up (maybe they missed the switch and wanted to tail your train)
After passing 7 avenue, the train would switch onto the uptown IND 8 avenue tracks and run express all the way to 200 street where it would go onto one of the yard leads
In the yard, the train would get onto the 1 train flyover and merge onto the 1 track. The train would then run down the center IRT Bway/7 Avenue track to 103 street then do a switching maneuver south of 96 street to the 2/3 tracks. It would run on the irt lenox ave. line to 145 street, and follow the 2 into the bronx. At any point on the 2/5 line, it could reverse and head down the 5 tracks into manhattan, get to the local 6 tracks, head to the westchester yard and voila, theres the next r62a 6 train.
As for the R33 cars, those were the work motors. Why they were there, im not so sure...
Its a major feat to relocate a 7 train set
MTA project completion date calculator Take the MTA 's estimate for completion and add 8 years to that.