i’m a high schooler, so i have a lot less knowledge than you guys, but as jd said, its probably from football
just looking at the games, there is rarely a chance to hit top speed, and accelrating is done from an upright position, so to compensate for the lack of tilt (compared to exiting blocks) they can achieve, while, say, taking a handoff, i know at least the football players at my school lower there hip height so they can add a little “push” so maybe they just cant get out of this habit