I play support a lot, and use grenades constantly, so I have noticed this. It usually gets me killed when it happens. What happens is you throw a grenade far away, but then it explodes like it is right next to you. After it happened a few times, I was careful to watch where my nade went in case I had bumped a stalker accidentally or something. Nope, I clearly see the nade fly far away from me, then it blows up and I die.