2017 ram 2500 broken manifold bolts

Dale Davies said

Two qualities to discuss; tensile and shear.
As the bolt grade increases the tensile strength increases through chemistry and heat treatment. This increased tensile strength provides higher clamping load, but at a cost, Shear strength. A gr 8 bolt will break in shear while a gr 5 or gr 2 bolt will tend to bend.
Hope that clears that up some.

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What is the source of shear load on the bolt in an exhaust manifold application? Considering the bolt is torqued to spec into a through hole with about .020" radial clearance, there should be nearly no shear or sideways force on the bolt, it's all in tension, no? Any force coming back from expansion of the pipe system that might cause shear would be absorbed by the mounting system and pipe deflection and anything else would be shared across all the bolts in the manifold. How significant could that force be?