it might be because the of the use and heat, the screwheads come slightly loose and shift the bed up.

i know mine shifts up by 1cm sometimes, even during a print. so i check on my larger prints every hour or 2, and spin the knobs a little to the right to shift the bed down just enough so it's not scraping off the previous layer

Maybe your springs are just old. On mine in order to go down which goes against the springs it takes a lot of force to turn. Enough that if I have to do it often it starts making blisters on my fingers. Going the other way is not as hard but there is enough force that I don't see how they could move on their own. As for thermal expansion, I don't think the bed gets hot enough to cause any measurable expansion in the springs, screws, or knobs.I mean at most you would run the bed at 110, that is like 220 or so in F. Not really hot enough to cause any real expansion in steel.

i know mine shifts up by 1cm sometimes, even during a print. so i check on my larger prints every hour or 2, and spin the knobs a little to the right to shift the bed down just enough so it's not scraping off the previous layer