If I'm orthodox, I throw my left vertical, but throw horizontal with my right in southpaw. Could be the height at with you throw it, as shovel hooks can sorta turn like uppercuts, could be how confident you are with your precision in each hand (I broke my thumb on a guy's head when he turned into my hook before I could snap it across)
Other factors could include whether you tend to overshoot or undershoot it. Russians almost turn their fist over to a backhand, like a casting punch, because if you go past the guy's head, it's easier to clinch up, so I could see people throwing vertical hooks because you could almost go right to the thai clinch if you're past their head.
Plenty of reasons really, a lot of it can just be personal comfort. I tend to think horizontal knuckles is the 'right' way if you're looking at power if you hit your spot correctly, but that doesn't mean it's the most comfortable for me. I personally believe a glock is gonna be one of the most reliable and practical sidearms you can own, but if I don't like shooting it because of it's shitty blocky design, it's not the right gun for me, no matter how good it is. Partial to sigs anyway
You just find your way of doing it to it's most effective, that's what martial arts tend to be
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