In questions where a wh-element refers to the object, we can observe SAI (Subject-Auxilliary inversion).
[Who did [you see]]?
As far as I'm aware, C-head has a [+Q] feature and it's occupied by a null bound morpheme that needs a verb to attach to. Since V-to-C is impossible (for the previous step, V-to-I is illicit as well) in English, only auxiliaries have the power to raise to C.
But why is it so that wh-elements pertaining to the subject of the clause do not trigger SAI? The C-head still has the [+Q] feature and do-support is available in English if there is no other auxilliary present.
[Who [read this book]]?