The answer to 'why is my shower overflowing' is that the drain is likely clogged, causing water to back up and overflow the shower. This can happen due to hair, soap scum, and other debris accumulating in the drain over time. Symptoms may include slow draining, gurgling sounds, or water pooling around the shower floor.
To fix this, you can try a few DIY solutions first. Use a plunger to attempt to clear the clog, or use a drain snake or auger to physically remove the blockage. If those don't work, you may need to disassemble the drain to clear it manually. However, if the clog is severe or the issue persists, it's best to call a professional plumber who can properly diagnose and resolve the problem.