Situation: you are a poor draftee being led by some other kid who happens to have a college degree (an officer). There are several of you, and one of him. Your living conditions are terrible and you're often asked to do things that might get you killed, and all of the reasons you're told you have to do this sure smell like bullshit.
Solution: you make your officer (another kid) piss-pants scared that if he doesn't lie and say you all totally went on that combat patrol you were sent on (rather than hanging around in the woods somewhere relatively safe for a couple hours, then walking right back to base) he might go to bed and not wake up because someone tossed a hand grenade in his tent and killed him. Or if he confiscates your drugs or hooch. Or tries to enforce grooming standards. Or anything else that makes your life worse. Basically, make him deathly afraid of upsetting "his men" in any way.
Grenades were the threat/weapon of choice because one tossed into a tent was pretty damn certain to kill the person sleeping there; deploying a grenade is very quiet (up until the boom, of course); they're much smaller than a firearm (easier to conceal even than a service pistol); and the slight delay between throw and detonation gave time to get some distance. In practice, it turned out to be extremely hard to prove who had committed these murders, especially if others in the unit weren't inclined to be honest about things they'd seen and heard that night.
This was A Thing toward the later end of the Vietnam War, and contributed mightily to reduced effectiveness of US ground forces. Turns out when you threaten people with death for no good reason they get kinda pissed off and murdery.
"Fragging" comes from "fragmentation grenade", or "frag" for short.