The main drain is plugged. Not completely, but enough that it is easier for the upstairs water to come out in the shower and floor drain instead of going out the drain.
Gravity. Also- If someone takes a shower with the curtain on the outside of the tub this can happen.
The shower drain and waste lines on up to the other fixtures located above need to be clear so that water will flow. Just downhill of the shower drain there is probably a blockage in the waste pipe. The blockage could cause slow draining. If it is completely blocked, the shower pan will not drain back out over time. You will need to run a plumbers snake down the line below the shower drain. If the toilet in the same room as the shower when flushed also shows water coming into the shower drain, it will be best to take up the toilet off the base and run a plumbers snake from that point. Warning for very slow draining lines or lines that do not drain, DO NOT use Drain cleaning chemicals. Good luck and you may wish to call rotorooter.
backing out- you have considerably less vision
It's because heat travels upwards and cold downwards so naturally the upstairs will become hotter then the downstairs.
It is much safer to take a bath than a shower cause in the shower you can slip so I would bath
cause you dont shower
Your kitchen sink drain and your shower drain are connected to a common drain line. There is some sort of blockage in the drain line past the point where the shower drain ties in. You need to call a plumber to run a snake through the drain line from the kitchen to where it ties in to the main drain of your house.
blown head gasket
Edema
Seats in the main valve body are leaking by, allowing water to go to the shower head.
I used to live in an apartment building that I noticed this problem...it comes from drain pipes connecting to each other, possibly not completely draining out, leaving used water in the drain pipes. I use to pour some bleach down the bathroom shower/tub drain....and the bathroom sink every couple of days before I left for work...fresh as a daisy when I got home!
yes cause it cant have a shower dah