I take a shower. When done I shut off the water. There’s a minimal amount of dripping from the shower head that stops after maybe 30 seconds. Then maybe 2 minutes later there is a sudden small burst of drips. Then no more.
This happens on two different types of showers in our house; one with just a pipe sticking out of the wall, and the other with a fancy hanging hose so you can take the shower head down and move it around.
The dripping is probably just water that’s caught in the showerhead dripping back out. The ‘burst’ at the end is (a big of a guess here) when an air bubble finally works it’s way up to the top of the (local) system and forces all the water out behind it. Remember, the water pipe goes up and then points back down to your head and there’s going to be water caught in that part of the pipe that’s pointing back down. Gravity wants to pull that water down, but it needs air to enter the system for it to come out. Eventually it happens, but it takes a few minutes.
backflow prevention device? These are usually required on handheld shower devices, though I would not have expected one on the “pipe sticking out of the wall” shower. This is a checkvalve that is forced shut when there is some small amount of water pressure in the line, but when the water pressure goes to zero, it opens to atmosphere, preventing the showerhead from sucking up unknown water back into your house’s (and the city’s) pipes in the event that the city’s water pressure goes to zero for some reason (e.g. local system shutdown for maintenance).
Possibly the preventer is sticking shut briefly, causing it to open in a delayed manner? Maybe it doesn’t open until enough water has dripped out of the showerhead to completely relieve any backpressure?
My guess: Water is condensing inside the shower head as it cools. At first, surface tension keeps the water from running out the small holes. When enough water accumulates, it is heavy enough to break the tension and it all flows out.
There’s too much water for that. It’s a good globule of water. I think your theory is partially right : when you turn the tap off, there’s still a bunch of water inside the shower head. It is connected through a column of water all the way to the valve. Air gradually bubbles up through the holes in the shower head, and water drips out in exchange.
Eventually, enough air gets into the growing air cavity in the shower head that there is a path between the cavity and one of the holes to the outside (the shower head) air. This means there is no longer a vacuum, and all of the water in the shower head can pour out at once.