Folks typically drain the water heater when they are not going to need
it for a while.
Some drain it at the end of every trip and some drain it only if it's going to
freeze or it's going to be a long time till next use.
Either way there is almost always a little air trapped in the system so
just because you have shut off the water pump or disconnected the city
water inlet- there still will be pressure in the system until you open a
faucet and let it out.
BTW- I also have pulled the plug without opening a faucet as many others have stated. It's a memorable experience but time dulls the memories and
makes me wonder if I'll ever do it again.