A couple reasons guys do this, for different reasons at different times. One, is drop sets and it's easier to take them off, but you said he wasn't doing drop sets.
Another is it's easier to take them off if the bar is on an angle.
If those two don't apply, it could be because he likes to hear the jingle jangle of the weight plates. I do this on a squat bar but it doesn't make much sense on a Hack Squat.
Or it could simply because the photographer asked for that as a variation of a picture.