It's a novice thing as well as a fear thing.
I've taken two male friends who'd never shot before to the range. Both of them are over 6 feet tall, with one at about 320 pounds and the other maybe 150 pounds, and they both leaned backward even after I demonstrated proper stance.
Another time I went with a so-called veteran shooter, also over 6 feet tall and about 230 pounds, to an indoor range. For a guy who'd been shooting a while, his stance was awful -- wide open, with I think the wrong foot forward -- and he actually shot while leaning backwards. So I made some suggestions and he improved his stance. Later he got into competitive shooting, I suspect more for male comraderie rather than for precision.
Anyway I'm guessing it's more of a fear of the firearm. Think about it: Some hunk of metal that seemingly and magically explodes in the hands ... Who wouldn't be scared of such potential harm?
Once a shooter understands that the firearm will explode in his hands and not hurt him; is taught proper stance, including maybe having his instructor place a hand on his back to remind him to lean forward; and becomes comfortable with the firearm and its recoil, he'll dispense with the fear and stop leaning backward.
"Let those find fault whose wit's so very small,
They've need to show that they can think at all;
Errors, like straws, upon the surface flow;
He who would search for pearls, must dive below." -- John Dryden