The decreasing values of bond angles from ${\text{NH}}_3$ ($106^{\circ }$) to ${\text{SbH}}_3$ ($101^{\circ }$) down group-15 of the periodic table is due to:

As we go down the group, the electronegativity of the central atom decreases.In ${\text{NH}}_3$, the bonding electron pairs are closer to the Nitrogen atom. This leads to higher repulsion between the bond pairs, which pushes them further apart (larger angle).In ${\text{SbH}}_3$, the bonding pairs are further away from the central Antimony atom, resulting in less repulsion and a smaller bond angle.