Just another glitch I've been noticing from time to time. Basically, what happens is that (occasionally) an enemy thrown semtex that sticks to your Shield will detonate instantly, rather than going through the beeping phase. Note that this happens at relatively close range, so long throwing distances aren't the cause of this phenomenon.
So yeah, I'm butthurt and want it fixed. Tweet this to Vonderhurr or whatever.
I'm sure they intended this. Just another way to screw shield users.
We used to be able to toss a trophy system down to save ourselves, but they patched it so if you do ANYTHING that involves moving the shield (tactical, lethal, change to your weapon) it instantly detonates.