I had this situation when fighting Miltiades thugs and perhaps when facing the raiders at Aemolas village.
The guy right in front of you may get knocked down if your shield bash is successful (THC roll) and you score a critical (CS roll) and the critical results in a knockdown (I don't know the percentages for different shieldbash+CS effects).
And he won't be pushed back even if you score this particular shieldbash effect.
The guy standing behind him won't be affected in any way.
The right answer is option 7: "Nothing will happen in most cases or the guy #1 will be knockdown in some rare cases".
Also, another point reg. shield bashing - you cannot knock back a knocked down opponent, and that's inconvenient for a Throwing+Block build.
Well at least cant be pushed back while knocked down part make sense, while can't be pushed back if someone standing behind him not so much.
It is probably assumed that the guy behind your opponent catches him, but it would be cool if there was a chance two push back two enemies at once.
Maybe it was difficult to implement or they did not had time.