My ‘theory’:

Your sphincter and other pelvic floor muscles functionally work together to hold your pee in.

You actually have two sphincters, the external one is skeletal muscle and is controlled voluntarily/directly by you most of the time, the internal one is smooth muscle, that you generally cannot consciously control, its on autopilot.

When you really need to hold your pee in, your sphincters get more active, and start to get tired.

The muscle groups near them, they basically try to do load balancing, by shifting you around into other configurations that make it easier for the specific muscles to be able to hold your urethra shut.

Thats why you find yourself, or may be directing yourself, to reorient/reconfigure your pelvic muscles, to basically ‘help’ your sphincters be able to hold on a bit longer, by finding a mechanically less demanding pose for them to assume, that still keeps the gates closed.

Think of how if you’re carrying something heavy for a while, you’ll tend to try and shift to a different way / configuration of carrying the thing, that uses different muscle groups in different ways, if a particular muscle group is just way too exhausted.