From Proto-Italic*-tōr, from Proto-Indo-European*-tōr < *-tor-s. The ō from the nominative case was made common to all cases. Afterwards nom.sg. -tōr > -tor, by Latin sound laws. Paradoxically, as in other r-stems (soror, -or), in the resulting paradigm the one form with a short stem vowel is the only form whose stem was etymologically long.
Cognate to Ancient Greek -τωρ (-tōr) as in δώτωρ (dṓtōr).
Compare Latin -trum (instrumental suffix), from Proto-Indo-European *-tr-o-m (instrumental suffix), from a separate ablaut *-tḗr (agentive suffix) (whence also -τήρ (-tḗr) as in δοτήρ (dotḗr)).