Most of the time this random occurrence is a result of poor or no venting of the fixture. Vents allow for air flow to the toilet sewage system. Without the vent, air pockets form in the waste branches and can cause what is called back-siphonage. Without a vent, the air in the system can only escape through the sewer and can pull the waste in the trap of the toilet with it. This is one of those problems that will require an expert, contact a professional plumber.