The decreasing values of bond angles from ${\mathrm{NH}}_3\mathrm{\hspace{0.17em}}\left({106}^0\right)\mathrm{\hspace{0.17em}}\mathrm{to}\mathrm{\hspace{0.17em}\hspace{0.17em}}{\mathrm{SbH}}_3\mathrm{\hspace{0.17em}}\left({101}^0\right)$ down group-15 of the periodic table is due to 

Decreasing electronegativity

Increasing bp-bp repulsion

Increasing p-orbital character in ${\mathrm{sp}}^3$

Decreasing lp-bp repulsion.