Of course it shouldn't happen at all on dropping the pressure if the
ball valve is working properly. My theory is that some imperfection
or dirt on the seating allows it to leak when the pressure falls just
below the resting mains pressure (presumably regulated at the stree
stopcock) allowing part of the diaphragm to lift slightly. But when
the pressure falls further the shape changes further and it seals
again at another part of the diaphragm radius.. If this is true,
there will be a rate of carefully adjusted flow from the tap which
will duplicate the effect of the shower.
So, a testable hypothesis!


Stop taps don't regulate pressure - only flow. The static pressure remains
the same.