Comic Strips

Not a plumber but I do have some experience with plumbing. This issue tends to happen with older plumbing methods where you run one main trunk pipe that you then branch into every fixture. Basically turning on the hot water farther up that trunk from the shower can cause a large pressure drop in the hot water supply at the shower. Basically the closer a fixture is to the source of the hot water, the more priority it has for the hot water supply. Also undersized piping can also make the issue much worse.

The problem can be mitigated in a few ways. By using low flow aerators on your sinks, you limit the maximum flow rate of water out of the sink and thus cause a smaller pressure drop when you turn the sink on. Having the shower be the first fixture along that plumbing trunk can also ensure that it gets priority for hot water. Also just increasing pipe sizing throught the system can help.

Newer style systems where you have a central manifold that immediately branches off to each room or fixture mitigate the issue because all of the fixtures share the water pressure more evenly. I've also seen some more expensive newer houses just have the bathroom fed by a seperate tankless water heater which eliminates the issue entirely because then it has it's own dedicated hot water supply.