Jindavik.
To answer your question the system used in these aircraft are automatic and the cabin differential is programmed to a maximum cabin altitude of about 8000 feet (at max cruise altitude of about FL410 for the 737-800).
If cruising at a lower level the cabin altitude will be lower.
Typically any pain experienced in case of inner ear infections/colds and/or eustachian tube blockages will be more noticeable whilst descending while the gases are trying to escape the body.
The system is programmed for around 300-500 feet per minute on descent which is quite comfy for the pax.
It sounds like the people affected all flew with head colds, inner ear infections or similar.
The B737 is the most popular passenger jet in the world so if this was an endemic problem with
VB all pax would complain which they dont.
Coincidence maybe, but whilst pax are warned not to fly with a head cold it is quite obvious alot of pax are either ignorant of the effects of flying as described or ignore warnings.