I cannot provide a definitive explanation. I only have my own anecdotal assumptions.
I have always assumed that the cats which do this are motivated by their prey drive and their "need" to hunt & stalk.
Cats differ as to how they express this need. A common way people experience a cat's prey drive is as an obsession to chase a dangling piece of string. A more elaborate cat toy designed to stimulate this same impulse are the wand toys such as "Da Bird" (example pic below).
Media figure Jackson Galaxy advocates regular "play therapy" with your cat(s) in order to satisfy what he considers an innate need to hunt. I tend to agree with this advice, though I don't know if it has ever been investigated (verified) in a formal fashion.
When you make the bed, the cats can be attracted by the sheets flying through the air and attempt to pounce on and catch them.
Another motivation can be either to explore or hide in the small "burrow" created when they get underneath the sheets.
Whether a cat is playing or hunting when they do this does not strike me as something worth debating. It is likely different elements of each for different cats.
I have several cats, but only one of them is especially interested in "playing" when I am trying to make the bed. I find it extremely annoying. (I also really miss it the occasional times he does not do it.)