Its a loss in water pressure to the shower.
You probably need new gaskets in your faucet.
Yes, it can also cross through a two handled faucet. Both the hot and cold sides of the faucet must be on and the flow of water must be stopped or greatly reduced on the outlet side of the faucet. This can occur when a hose with a spray nozzle is attached to a laundry tub faucet with the faucet on and the spray nozzle off, or when a shower faucet is on but the shower spray head is off. When another cold water outlet in the system is turned on then the pressure on the cold side of the laundry tub or shower faucet in question could be lowered below the pressure on the hot side and hot water can flow into the cold water side. There are faucets which have integral check valves which prevent this from occurring. WWW.WOODBRIDGEPLUMBING.COM
Have you got two single faucets or a single lever faucet? It could be a pressure imbalance or it could just need maintenance
It doesn't but you better run sir/madam their may be a ghost in your bathroom.
They are probably not strapped in the wall. If you have a quick closing faucet, there may be water hammer.
You need to check your hot/cold handles or the shower spicket. Not sure what type you have, but they come off. More than likely you need to just replace a gasket..
The faucet drips even when the water is turned off.
Seperate valve for outside faucet turned off, broken valve stem inside of outside faucet or handle stripped at faucet.
If you have an instantaneous hot water heater, and your faucet doesn't flow enough water it won't turn on the water heater. Thus requiring another faucet to be turned on.
Never happens.
You don't say whether you are referring to a single handed mixer or an individual faucet. If a mixer then the o rings inside have probably worn away and need to be replaced. If a single faucet then either the washer needs replacing inside the tap or the gland nut (if have one) needs to be serviced. Dont forget that in a single knob faucet, there is also an adjustment to be made for the hot water temp. that comes from the shower when turned to hot....it is a sort of dial in system remove the knob handle with a Philips screw driver and see where the settings are if too high adjust down a few clicks(notches).
I bet somewhere in the faucet the installer used a black iron nipple for the spout or shower head. The nipple rust very quickly. A few seconds of water rinses it out. The nipple will start leaking in the future.