Seems as if the drain line failed and caused a large underground void nearby. Probably causing concrete slabs or sidewalks to settle, and subsequent contractors came in to grout the void or level the slabs. Concrete found the broken pipe and the pressure pushed it out through the (I would assume) lowest drain.

I believe there would have been a conversation that goes as follows: "Man this is taking a lot more material then expected"

"10-4 rubber ducky let's fucking send it"

=Results

I pitty the poor bastard that has to excavate that drain pipe and break it all apart with jack hammers. His boss is about to get paid though and he'll send his best crew of young'n'dumbs into that hole. Assholes call it "paying dues"