The 1908 event I believe was so devastating over such a large are because it was like an airburst, like the biggest shotgun blast ever. This one left a crater so there had to be something substantial in its composition, enough dense material like iron to survive the atmosphere but small enough to not create a super-massive crater, could have been a chunk of iron no bigger than a brick. And I think your companion theory may be right, the asteroid belt is one big shooting gallery, all you need is one thing to ding another and its a pinball effect.