At the times I wasn't in iron sights but I had pressed it soon before the incidents.
The 2 most recent times this happened was 1) was on a very low pop server and I decided to YOLO my way into NW Airfield tents. I aggro'd a bunch of zombies while running in 3rd person so that I could kind of gather them all together and shoot a bunch at once. So after running for a bit in 3rd person, I stopped, turned, right mouse buttoned into ironsights to shotgun blast 4 of them. Then exited ironsights and lowered the gun with spacebar, turned and kept running as more were starting to aggro and I needed to reload.
When I started running in 3rd person after this I started to get the rubber banding back and running on the spot. If I raised the gun with spacebar I was able to move freely again, but I was still in 3rd person and not looking down the iron sights.

The 2nd most recent time this happened was last night and I suppose it was the same situation. I was looking for a character playing in 3rd person, hid in some trees, scoped in with RMB to try to get a visual, then exited again to run to a different location, encountered the player but began running on the spot almost immediately, so I hit spacebar to bring the gun up and escape the running on the spot while he shot at me, then turned and tried to hip fire shoot and died.

Should I expect this to happen every time you use the RMB, or is it reset if you put your gun away then take it back out again? It's something I could experiment with I suppose but I'm not sure how much experimenting others have done with this. It was hard to search through the bug report list.