The answer to 'why is my toilet overflowing' is that there is likely a clog or blockage in the toilet or drain line. Common causes include flushing too much toilet paper, foreign objects getting stuck, or a buildup of waste and debris. Symptoms include the toilet water rising and spilling over the bowl when flushed.
To troubleshoot, first try plunging the toilet to dislodge the clog. If that doesn't work, you may need to use a toilet auger (also called a plumber's snake) to clear the blockage. If the clog is deeper in the drain line, you may need to call a professional plumber to use a larger snake or hydro-jet to clear the line.