Hard to say but here's a thought.
Years ago I had some service done on an 1988 Camry at a Toyota dealer, a CV boot replacement if memory serves. Shortly after I would hear a loud clunk when shifting to reverse, but only when the vehicle was pointed uphill and always under that situation. I would have thought it was a trans "uh oh" had I not just had that service. I took it back in and they couldn't find anything wrong, perhaps because the service writer omitted the "uphill" variable or the tech overlooked it or pooh poohed it. I then asked for a tech to get in the vehicle so I could recreate it and there happened to be an incline right there in the dealer parking lot. "Clunk."
Hearing the clunk, the tech says, "I know what that is." 10 minutes later I'm on my way and it never happened again. It was a loose bolt. Where I cannot recall but if it was a CV boot job it stands to reason it was somewhere in the front suspension that they failed to torque properly.
The moral of the story is that since you only encountered this once over a couple of months under very specific circumstances, it would be hard to recreate and you're likely to get, "we can't find anything wrong." If that happens, ask then to check all of the bolts in the undercarriage--perhaps the rear suspension or differential given you felt like you were hit from behind. It wouldn't be the first time a bolt (or bolts) were not property torqued at the factory. Some have reported new CX-5's with the trans fluid underfilled to take one example.
Admittedly, this is a shot in the dark.
