josh, this happens all the time on a small, small scale with drain cleaning
it happens when when you have a stoppage and then it clears downstream.
usually when i have a main line stoppage, and i clear it, a surge of waste water starts to flow downstream. if the line blocks downstream, which is very common with roots that break off and plug the next section, the flow will come to quick stop. this will create a water hammer affect. thus backing up the sudden surge of waste water. sure it's not a geyser, but then again i'm not clearing manholes with my k-60 either