John Gill Commentary 1 Kings 11:30

"And Ahijah laid hold of the new garment that was on him, and rent it in twelve pieces." — 1 Kings 11:30 (ASV)

And Ahijah caught the new garment that was on him
This looks as if it was Jeroboam's garment, having got a new one to appear before the king in; though the sense may be this, that the prophet took hold of his own garment that was upon himself:

and rent it in twelve pieces ;
as symbolical of the twelve tribes of Israel.