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

The decreasing value of bond angles from NH_3​ (106^∘) to SbH₃​ (91^∘) down group-15 of the periodic table is due to decreasing electronegativity.
 

As the electronegativity decreases, the bond pair of electrons becomes farther away from the central atom and the repulsion between the bond pair of electrons decreases. 

Due to this, H−M−H bond angle decreases.