broken ring gear teeth

what would be the cause of having two broken gear teeth on the rear differental ring gear after 130,000 miles of mostly highway drving,no towing and following
maintainance service intervals ,other than having a 95 honda passport/isuzu rodeo 4WD w/posi rear ,dont have a clue other than it being made by isuzu,reliable up till now but i think its time to get rid of it,just wondering what would do that.!!!!!!!
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