In a lot of cases it’s because it’s easier to look up, especially when a load is being hoisted by crane, without having to look under the visor (even as short as it is).
In other cases, workers who have to wear flip-up face shields when working with grinders find it a better fit to attach the shield so that it doesn’t have to clear the visor when raising and lowering it. (Plus there’s a smaller opening at the top of the face shield that way.)
In addition, if the entire hat is turned around (meaning the suspension as well as the shell itself), then a lot of modern hats that have an adjustable fit have the knob to adjust the headband tightness right at the forehead instead of the back of the neck, where it can be harder to reach.
And some just do that because they’re told not to.
In addition, I would not expect that it’s “a majority” who do so. Perhaps a majority of those you have seen, or a majority of those on several sites that you’ve visited, but I highly doubt “a majority”. For one thing, workers in Asia – where hard hats are worn at all, anyway – are generally required to be very uniform in appearance, right down to wearing hard hats “to the front”, and including chin straps, as well.