The scrotum is one of those mystical body parts that has a mind of its own. You can’t control what it does and it does what it wants at the oddest times. In the summer I sometimes go to a local nudist park for a weekend. I can sit around my trailer all morning with the lowest hanging sack I ever have. But when I open the door to go to the pool or take a walk on a nature trail, where others will see me, my scrotum tightens up and pushes my flaccid straight out ahead of me and makes it look like I have the smallest flaccid there.
I don’t think there’s any scientific explanation for your condition DonGiovanni except that the testicles need a certain temperature to produce viable sperm and if the get too hot, the scrotum relaxes to allow them to cool. Perhaps hours of lovemaking gets your balls too hot and they need to hang lower to make more sperm. Whatever the reason there’s no need to worry and nothing you can really do about it.