Kinda late here, but I experienced "case removal yank-n-crank" as well when I first got my 75rt......on one side only....what?
**Disclaimer** I must be the only one on the planet that owns this unknown-to-me brand of tail rack and it's mounting scenario....so this may not apply, but maybe there's some sort of "spacing" issue between frame and Z Bracket that's affecting your installation of your bags.....
I have a Non-Stock tail cowl top box rack that shares a mounting point with the Z bracket and after removing the rear fender-license plate plastic thing, and taking a good long look, I found the top mounting point of the Z-bracket, for the bag I was having difficulty with, had been reassembled differently than the other side..the other side coming on and off smoothly......i.e., the P.O. had at one time taken the Z brackets off, and upon re installation, had positioned the Z bracket on the "inside" of the bracket to rack to frame mounting relationship. Kinda hard to explain, and I dont have a pic, but by moving the Z bracket's top mount to the "outside" of that stack - ---from the outside-in would be: bolt, washer, Z bracket, rack mount, frame ....it allowed the Z bracket it's proper spacing and the upper tab of the bag would now fit with out the fiddling.
By being on the "inside" of the rack mount/Z bracket sandwich, the problem was it was pinching the bag mounts tab, me having real trouble working the bag on and off. Take a look at yours.. you'll have to remove the rear plastic fender piece to observe and adjust, if necessary.