I don't think there's any kind of grenade magnetism. The problem comes with the blast radius, grenade properties, player movement and shield/health system.
The blast radius is big but the further from the epicenter you are, the significantly less damage you take. This seems like a good idea but with a big radius like Reach nades have in conjunction with the health system... bad idea. Reach shields take a LOOOONG time to recharge to full capacity, and if you take the SMALLEST amount of damage, like a sliver of shield from a grenade shockwave, then your shields will stop recharging and you have to wait again. I've gone like 25 seconds at times without being able to recharge my shields. Even if they're 2/3's of the way recharged, if a nade falls at your feet You Are Already Dead.
Then, the health system under it. If you're red health, full shields, and close to a grenade, You Are Already Dead.
The other problem is that grenades, very inconsistently, can chain react with grenades on the ground. At least I'm like 90% sure they do, because nothing else explains the multiple explosions when only 1 or 2 grenades are thrown.
But perhaps the biggest one is player movement and the grenade fuse. The Reach grenades are just as powerful as the CE nades. The difference is in Reach, they have a significantly shorter fuse and you can't escape them once they hit near you thanks to a really stupid inertia system. There's been very mild inertia in Halo 3 also, but Reach takes it to another level. It makes strafing gimped and it prevents you from effectively getting out of the way of grenades.
I've been wondering what the game would play like if grenades acted like Reach melee, in that if you have any shield left, grenades would ONLY take out shields, not touch health. Then a second nade is necessary if you want a nade kill. Grenades in Halo have never really been meant to be used as weapons (besides stickies) They are mainly for flushing people out, sealing off an area temporarily, and most importantly, STRIPPING SHIELDS.
No matter how balanced the grenades may seem to be on paper, and maybe they are and we're overreacting, but most people agree that something about them feels off. They just aren't fun.