as you sit more upright, your arms rotate outward, to the front...

if your arms droop down below about a 90* to your spine, shoulder loading increases, your lower spine has less flex room, etc...
it's a geometry thing.
your once fitting bike becomes a cramped up, too short bike as you sit more upright, compounding lower back strains/pains..
those adjustable stems end up being the worst thing ever for "comfort", and even make the bikes feel twitchy when raised too far, IMO.

Walmart level beach cruiser/trike bars with WAY Too Much backsweep cause a similar problem, as well as making the wrist Angle painful.
i see an incredible amount of those bikes that are rarely ridden... and the bolt upright seating position also puts too much weight onto the butts, causing the "i need a better padded seat!" syndrome.. that causes nasty CHAFING of some very tender territory... i swap on far flatter bars, a mid width MTB seat, and instant Smiles are the typical result.... and a side benefit is easier standing up for hills and dismounting too!