Interference at first base

[RustyWinslow]

Dixie Youth 9-10yr. old
Ground ball to F1, throws to back pedaling F3 whose momentum carries him across the bag as if he was a base runner. Simultaneous collision on top of the bag at first, ball drops out, field umpire calls him safe, Home plate umpire (without anyone asking) promptly overrules, calls "interference" and tells me to go look at rule 7.09.
BR ran directly through the bag and never veered outside the double base path lines.
Is this not the wrong call (stating that rule) and what are the guidelines on stepping out and overruling the field umpire without being appealed to by either team.
Thanks (probably a stupid question, yet would appreciate the help.)