Why do sewer pipes overflow?

It depends on whether the tank or bowl overflows. In either case: Immediately open the tank (remove the lid)& lift the float, or valve arm, or ... (if you are reading this, I'm probably too late to prevent the flooding.) Then turn off the valve at the wall under the tank & put a small bucket under it, because this shut-off valve often leaks a bit after being turned. If the tank: Fix the overflow (standpipe), or the fill valve. They can be easily replaced by a handy home-owner (A bad flapper valve would not cause an overflow, but only cause the tank to leak into the bowl. If the bowl: Clear the clog in the sewer piping. Don't put big or water absorbent things down the drain, or else talk to your doctor.

Be careful working with backed-up sewage. Keep your hands and face clean and dry as possible. First, try an ordinary plunger, which is designed to PULL A CLOG UP (once loosened, it might wash away, or be up where you can remove it). Next, try a closet (sewer) auger [buy or rent], which is intended to break-up, or snag and extract the clog. You can sometimes get an idea how far down the clog is, by which other drains on the same or lower floors are backed up.

If still nada, I would call a plumber since if these don't work the obstruction is probably pretty far down the line.

If the main house drain is clogged, there is usually a 4" or 6" clear-out plug near the lowest point of the building sewer Plumbing. It WILL GUSH when opened (unless it is at the bottom of a tall dam-pipe). It is intended to allow you access to a clog that has settled in the main gas trap (a water collection point before the main sewer line to the community line (usually in the street). There might also be another clear-out plug in the yard, near the building, for inserting a rotary router, usually to clear out tree roots that have invaded your sewer line thru a crack, for the water and nutrients.

There are other sites that describe how to use the various plumbing equipment you might need.