The bond angle in ${\mathrm{XeO}}_3$ molecule decreases from $109°{28}^1$ to $103°$. This is due to

• ${\mathrm{XeO}}_3$ that is, xenon trioxide has a pyramidal geometry. ${\mathrm{XeO}}_3$, contains three bond pairs (three oxygen atoms doubly bonded with xenon) and 1 lone pair of electrons.

So, here lone pair-bond pair repulsion takes place due to which we see a decrease in the bond angle from $109°{28}^1$ to $103°$.