What I’ve heard is that it often occurred in rear areas, not necessarily that it was rear lines personnel. Looking into it now that it seems you’re right. To me it sounds like a similar situation to personal acts of racism in Vietnam. Those on the front lines had bigger things to worry about, and had to rely on each other. In the rear, tensions could escalate over disputes.
One book I’m adding to my wishlist now is Fragging: Why US Soldiers Assaulted Their Officers in Vietnam by George Lepre.
An article in Foreign Policy summarizes some of Lepre’s findings. This includes: “Most fragging occurred in the noncombat support units in the rear, not in front-line combat units. (p. 31)”