If your shower (or any drain) is draining to your sump pit, you are overworking your sump pump and shortening it's life span. If you have a radon mitigation system that uses the sump pit, you are also creating a situation where radon can potentially enter your house. The only water that should ever be in the sump pit, is ground water.
Aside from that, here are a few things that may cause your issue.
Evaporation
If the shower is not used very often, the trap can naturally dry out due to evaporation.
Solution
Either use the shower more, or routinely pour water down the drain.
Blockage in dry vent
If the vent becomes obstructed, water flowing down the drain can produce a syphoning action and suck the trap dry.
Solution
Clear the blockage in the vent, by snaking out the vent pipe.
Blockage in wet vent
In situations where a vent is not directly connected to a drain line, a wet vent will be used. A wet vent, is a pipe that serves as both a drain line and a vent.
If this pipe becomes restricted, syphoning action similar to a blocked dry vent can suck the trap dry.
Air cannot get past the obstruction while water is flowing through the pipe, which creates a pressure difference that must be equalized. There are two things that can happen in this situation. Air can be forced past the obstruction (possibly causing gurgling), or air can be forced through the trap (causing gurgling that you're more likely to hear).
Solution
Snake the drain and clear the obstruction.
If you really are draining to a sump pit used in a radon mitigation system
If both drains and a radon mitigation system are connected to the sum pit, the radon system could be sucking the water from the trap. A radon mitigation system works by sucking radon out from under the house, and venting it outside. If the pressure in the radon vent is lower than the pressure in the house (which is sort of how they work, so it probably is), the water in the trap could be forced out and down the drain by the higher pressure air in the house.
Solution
Don't drain to the sump pit, especially if you have a radon mitigation system.
Best Answer
This is really a comment but too long -
I am not sure of your exact situation but normally if something like this happens you call your real estate agent, police, and bank. You need to file a police report, your real estate agent needs to get a hold of seller's agent (who is ultimately responsible in most states), and you need to talk to bank about freezing any money if you can. It might be too late for this but I don't know. But whatever it is the previous home owner (if can be proven) has committed a criminal act. Not taking action right away hurts your case.
There is no magic to get rid of stuff like grout in pipes. First any serious chemical I might suggest may damage the pipes. Second I am not even sure it is grout in your pipes - it could be various types of cement or thinset too... Then I am not sure to what extent the material got in. This could go all the way to the street which may cost a lot of money - I have seen a similar scenario run home owner 15K done cheaply.
Also most insurance companies don't cover criminal acts that aren't reported. But the issue you have here is that this probably happened before you officially had insurance (that is why I mentioned getting agents involved). If you have some sort of new home owner's insurance that might cover it but they will basically be going after the previous owner for you.
Right now you need to file a report, take a TON of pictures and video and have a plumber come out. You can't start hacking everything up or you are making it worse. The plumber will be able to scope the line to see what the extent of damage there is ($200-300 tops). The last thing you want to do is repair something incorrectly or cause more damage (overflow) and have it blamed on something you have done.