Question

Why does water flow decrease in your shower when someone flushes the toilet ​

Answer 1

The water from the toilet flush takes a lot of the water power out of the system which leads to a decrease in the flow of water to the shower. The water system is connected.

Answer 2

A toilet requires a large amount of cold water to flush. So, when the toilet flushes while you're in the shower, it's stealing from your shower's cold water supply. When the pressure-balancing valve senses the drop in cold water pressure, it responds by restricting the hot water pressure.