Sir Richard C. Jebb, Commentary on Sophocles: Oedipus at Colonus

ἔμπολιν: cp. 637. As Theseus was returning from the rescue, word had been brought him that a stranger had seated himself as a suppliant on the steps of the altar of Poseidon at Colonus (see on 55). This man said merely that he was a kinsman of Oedipus; and that he wished to speak a few words to him (1162). The fact that he was not from Thebes, but from Argos (1167), seems to have been inferred from something in his dress, for Theseus says that he does not know whence the man had come (cp. 1161). Polyneices took this precaution of becoming a “ἱκέτης” because he did not know what power might now be at the command of the paternal anger which he foresaw (cp. 1165).

προσπεσόντα πως: lit., “"having somehow rushed to"” the altar: i.e. he had come in the absence of those Coloniates who had hurried from the sacrifice to the rescue (899), and no one had witnessed his arrival. (Cp. 156 “προπέσῃς”, 915 “ἐπεισπεσων”.)

πως could not mean, “"for an unknown reason."”