the reason a single grenade can kill you in reach is because the impact damage takes off enough shield for the explosion to kill you. it sucks
Actually the reason is usually because of the health system. If you have less than 100% health with full shields you can die from one grenade.
You may also be able to die from impact + explosion but I've never noticed it. I thought the damage from a grenade impact was relatively small.
EDIT: I am curious about the answer to this question about grenade behavior though. Can they be detonated when "dormant" or by bullets at all for that matter? Personally I didn't like the feature in Reach.