If your toilet is connected to a common drain I have seen the situation
where someone upstairs (perhaps several) dump enough water through the
drain to cause a suction effect that triggers the toilet to siphon.

If the toilet is filling up there is a drain obstruction on the sewer
main side of the pipe (or the pipe size is wrong) which allows the
sewer pipe to fill up. When the level is above your s bend the toilet
fills.

I am suprised you aren't getting water from the floor drain or the
shower or tub.

You need a plumber or your local plumbing inspector to look at the
setup and make suggestions.

You may be able to fit a backflow valve on the sewer line from your
apartment to the main.