My plumber did the same kind of install, and when he removed the old shower p-trap attached directly to the main drain the fernco coupling metal strap snapped open and either that or when he was shovelling under the drain caused a small hole in a water supply line running directly underneath the main drain (nobody knew there were water lines underneath, he wasn't sure which caused it).
He said when water first appeared in the hole that it is probably just ground water, gravity will take care of it - and went off for his lunch break - as you can see from the first picture, gravity didn't do much good, as it filled up with even more water (this was an hour later after he came back from lunch)
Lost half a day while the city (to their credit they responded in 30 minutes to an emergency flood call) tried to find the main shutoff outside the house - picture 2, took over an hour because it was actually buried under earth in the neighbour's yard, plus a lot of trial and error and head scratching from what to do because there were four water lines into the building - net result, ended up with gradually increasing water level in the excavation space (had to shop-vac it out completely, luckily it couldn't go above the level of the main drain which was open).
You might want to wait and see if it gets more water in the hole. The plumber had to fix the leak with a compression fitting.