Extreme shudder after rotor replacement?

Has anyone heard of this? I have a 92 318i. I had my brakes checked and the pads were fine. The rotors had hot spots though and the shop advised having them turned. After taking them off though, they found they were all under spec and had to be replaced. The front ones replaced were no problems, worked perfect. The back ones were replaced seperately and something happened that freaked me out. I left the shop and about 3 miles away, I felt a shudder start from the back and go through my car that was almost a rocking motion. I was driving straight and going about 25-30mph. It went away, but I didn't do anything but continue driving. It was not a imbalance or alignment type of shake or shudder. I went back to the shop and nothing happened so I couldn't get the tech to understand. I drove it back home and sure enough about 3 miles away it happened again, nearly the same circumstances. I was very afraid to drive it after that, thinking that something major was being damaged. The brakes were looked at and nothing was out of place or incorrect. It hasn't done it since and I've gone nearly 80 more miles. Has anyone a clue of what this may be? Could it have been the calipers self-adjusting? or grabbing? or over-adjusting? or the something on the brakes trying to re-seat? I'd appreciate any ideas. Sorry for the essay.