भावकर्मणोः

An आत्मनेपद affix is the substitute of the affix ल 3.4.69 when it denotes the action of the verb or the object of the verb.
Sutra 3.4.69 declares the "letter ल (लट्, लिट् &c.,) is placed after transitive verbs in denoting the object also as well as the agent; and after intransitives in marking the condition (i.e., the action itself which the verb imports) also as well as the agent." The Paras. and Atman. affixes would have come, therefore, generally after all these verbs. The present sutra restricts Atman. affixes to bhava and karma verbs.

This is clear. Impersonal verbs and passive verbs as a rule are conjugated in the atmanepada and take the terminations of that pada. These originally were verbs expressive of states rather than of actions. As of भाव we have ग्लायते भवता 'you dislike' (lit. 'it is disliked by you'), सुप्यते भवता 'you sleep.' Similarly passive verbs as क्रियते कटः 'the mat is made'; ह्रियते भारः 'the load is carried.'

All verbs in Sanskrit may have three voices, viz, active, passive, and middle or impersonal; active voice is generally parasmaipada. The reflexive verbs known as कर्मकर्तरि are also atmanepadi. Thus लूयते केदारः स्वयमेव 'the wood cuts of itself.' See sutra 1.3.78.