If the bolts holding the payload in place shear away under the lateral force of supporting the payload, the payload may fall and hit the hard inside of the launch vehicle, making the two events quite similar. (Indeed, the original drop I linked to occurred when the satellite was not correctly bolted to a table on which is was going to be tipped to one side in order to verify that it could withstand the stress of being in that physical orientation.)
That said, ugh's comment suggests my reasoning (as an explanation for vertical assembly) is either incorrect, incomplete, or both.
That said, ugh's comment suggests my reasoning (as an explanation for vertical assembly) is either incorrect, incomplete, or both.