Their are a couple answers to this question, but this solution fixes it 99% of the time. Poor venting 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 witch can cause what is called back-siphonage. The air in the system which, without a vent, can only ecscape through the sewer. It can pull the water in the trap of the toilet with it.
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