I think the bricks drop because the guardian you killed had special ammo.

Allow me to elaborate. -- I run Blast Furnace & Recluse. I don't use special ammo. I can't pick up special ammo. yet my teammates have reported that I still drop special ammo. The 5 or so seconds after you die, you can kind of spectate who killed you. I've seen a guy shotgun me, slide over my body, and then reload his shotgun (which implies that I dropped special). But thought the rule was you don't drop special unless you're using a gun that uses special.

Bungie changed something recently that has made people with double primaries drop special ammo. I only noticed because I specifically went double primary in a match because opposing team were using their specials as primaries, and I wanted to deprive them of ammo. So imagine my surprise when they kill me and I die, dropping special ammo. :P Other people have noted the same thing, and it's a relatively recent change.