The decreasing values of bond angles from NH₃ to SbH₃ down the group 15 of the periodic table is due to

The H - M - H bond angle is less than the original 109⁰, tetrahedral bond angle (H -N -H) in Ammonia 106⁰
because of greater repulsion between one pair an a on pair than between two bond pairs of electrons. Because electro-negativity of M decreases from N to Bi, the bond pair lie farther away from the central atom, and the lone pair causes greater distortion of bond angle. Thus H-P-H bond in Phosphine.