I figured it out myself tonight while playing an awesome game with few players on Oman. Here's the thing:
If you ignore the visual effect of the detonator being pulled out and simply go through with the motion, you can detonate your C4 packs almost immediately after tossing them. Try clicking Mouse1-Mouse2-Mouse1 really fast and you'll get it right. It will look like the C4 is being thrown a second time, but in actuality the first C4 pack is detonated and the second is still in your hand.
Messed me up too for a while, but now I can whoop some *** with C4. Watch out, I'ma comin!