I'm currently reading a Ph.D. thesis concerning the structure of English adjectives. These generative trees are used to illustrate the structure of the phrase "someone tall". The author states that Picture 185 is illicit since it violates HMC and Picture 186 is the correct one. So what is HMC and why it doesn't allow the N-to-D movement in this situation? I'm confused.
This stands for "Head Movement Constraint". It was introduced by Lisa Travis in her 1984 MIT dissertation Parameters and Effects of Word Order Variation, p. 131:
Head Movement Constraint: An X0 may only move into the the [sic] Y0 which properly governs it.
(Note that X0 is notation for an X head.)
In your (185), someone could not move from N to D, because the A head intervenes. For this reason the AP must be taken as an adjunct of the NP, so that the D does properly govern the N.