Why do nonmetal oxides usually form acids when mixed with water?

Nonmetal oxides are typically covalent compounds that react with water to form oxyacids, which subsequently release hydronium ions ($H_3{O}^{+}$) in aqueous solution.12

The reason for this behavior lies in the nature of these oxides:

• Acid Anhydrides: Many nonmetal oxides, such as sulfur trioxide ($S{O}_3$) and dinitrogen pentoxide ($N_2{O}_5$), are classified as acid anhydrides. When they react with water, they combine directly to form an acid without changing the oxidation number of the nonmetal atom. For example:1
  $$N_2{O}_5+H_{2}O\to 2HN{O}_3$$
• Disproportionation: In cases where the nonmetal is not in one of its common oxidation states, such as nitrogen dioxide ($N{O}_2$), the oxide may undergo a disproportionation reaction. In this process, the nonmetal is simultaneously oxidized and reduced, resulting in the formation of an acid.1

While most nonmetal oxides are acidic, there are exceptions, such as carbon monoxide and nitric oxide, which do not form acids when mixed with water.2