The affix ṭa comes after the verb sṛ 'to go' when pūrva in composition with it, denoting the agent.,
Thus पूर्वः सरति = पूर्वसरः 'going in front'.
Why do we say 'denoting the agent'? Otherwise the affix अण् will be employed. As पूर्वदेशं सरति = पूर्वसरः 'going to the east'.,