Question

why are the oxides of halogens mostly unstable?

Answer 1

As oxygen and halogen are both negatively charged meaning both are electro negative than the oxides of halogen gets to the extreme negativity as both carries the same charge and gets unstable.  

Answer 2

Explanation:

Bromine-Oxygen bond in its higher oxides are least stable among other Halogen-Oxides because, it lacks both: Higher Polarity (Br 2.8 vs 3.5) as in the case of Iodine and Multiple Bond Formation involving the d-orbitals as in the case of Chlorine.