Sure there can. Behind the shock wave is a massive amount of turbulence--that's why it's a mushroom cloud, not a hemisphere cloud (again, it shows a hydraulic jump). There's also the interactions of the material in the air--remember, not everything moves as fast as the shockwave (in fact, it can't, otherwise it wouldn't be a shockwave).
You have two options: either everything moves exactly as fast as the shockwave, which domonstrably doesn't happen (the stem of the mushroom cloud proves that), or turbulence is a major factor in the deposition of debris. Which is it--the impossible, or what you disagree with for no reason other than your continued mere assertions?