I bought this rehabbed home about a year ago. I live in a single frame home that has an overhead sewer (sump pump, ejector pump). The toilet on the first floor would overflow with sewage and it would come out fast. Nothing on the second floor was ever affected. I had a plumber who said not to flush baby wipes(lesson learned). But a month later it happened again with nothing clogging it. Same thing, the sewage water would shoot out of the first floor. Had a different plumber come out(since the first one clearly did not resolve the actual problem). He thought I had a clog but I never did when he checked it. He ran some kind of snake through the toilet that was leaking water after he shut off the water from the toilet. He said problem was fixed. He did mention if it happened again that he would have to put a camera to make sure there were not any breaks in the pipes outside of my home under the concrete. Sure enough it happened again. 

Note:

It happened at random times, once I was using the shower on the 2nd floor, came back down and it was the sewage from the toilet on the second floor only. 

The other two times it was random. 

It was so much water it dripped down through my floors and through vents and it was about 2 inches of water in the basement. 

I had a guy come out again and he ran a camera and said there was a "break" 5 feet from the home and he would have to rip the concrete to get to it.

Does anyone have any idea what may be going on?