It's linked to the grace period team mates have to revive their fallen comrades where a hit box forms over the fallen soldier to protect them from bullets. Trouble is, it protects others.
If a dying soldier lies beneath where you shoot then those bullets are prevented from hitting their target. The bug is more prominent with narrow corridors like in Operation Métro.
The 'Death Shield' does more than stop bullets however as other projectiles get batted away, as YouTuberexplains in one of his latest videos exposing the bug. Grenades for example will bounce off the hit box of a player as if they were still standing, and not lying on the floor prone.
DICE is now "looking into it" according to a from a QA Lead.
DICE "looking into" Battlefield 4 'Death Shield' bug
14 April 2014 | By Simon Priest