Stepping on plate an out ?? (LL)

[peterN]

Was at a Little League game the other day, and the Plate Umpire called a kid out for stepping on home plate after bunting the ball. The kid did stay in the box until the ball was hit, but stepped on the plate on his way to first.

I'm a dad, not an official, and tried to track this down in the Little League rule book a few years ago, and after the play the other night, I revisited the rule book (again) to no avail in my search for this as a reason to pit the runner out.
Is this an out for some reason that I am missing?

Thanx in advance for answers - peterN