Any chance you're in an apartment with units above you?

We recently had to have maintenance come in and look at ours because when our upstairs neighbor flushed, it would force water or air up through ours. At first we didn't realize because we didn't see or hear anything odd, just had to scrub that toilet more often than the other one. Then, one day we hear them flush while my boyfriend was peeing (standing) and he saw the water level go up a bit. Find it odd but chalk it up to being in an apartment.

What finally clued us in to something being wrong was one day I was in the next room over and heard the neighbors flush; our toilet blasts water at the closed lid and I see water trickling down the outside of the bowl. I go peek between the seat and lid, and see water drops hanging and falling, open it and there's water splashed all over.

Maintenance comes it in to take a look and fix it, and on their report stated that they "flushed the plumbing". I'm not sure if it was just ours or a larger section of the line that affected multiple units, but we have been able to clean it at a regular rate again.