The affix ṇa is used after the verbs du 'to burn' and nī 'to lead', when used without a preposition.,
Thus दु + ण = दौ + अ 7.2.115 = दावः 6.1.78 'burning'. So also नी + ण = नी + अ = नायः 'leading'.
Why do we say 'when used without a preposition'? Observe प्रदयः 'burning'; प्रणयः 'friendship'.,