Kindly explain this that why they disproportionate in acidic medium.

"All oxidation states from +1 to + 4 of nitrogen tend to disproportionate in acid solution."

Disproportionation reaction refers to the increase of oxidation number and as well as decrease in oxidation number of same element in the reaction.

In other words, we can say that same element is oxidised and reduced.

In case of Nitrogen, its oxidation number varies from -3 to +5. 

If Nitrogen has any oxidation number out of +1 to +4 in any compound then it has possibility to increases its oxidation upto +5 and also can decrease upto -3.

Hence, all oxidation from +1 to +4 of Nitrogen tend to disproportionate.

for example, in the following reaction, HNO₂ in which N has Oxidation state +3 is oxidised into HNO ₃ where N gets oxidation state +5. At the same time, HNO₂(N has Oxidation state +3) is reduced into NO where N gets oxidation state +2, hence reaction of HNO₂ shows disproportionation.