Because he keepeth not the sabbath (οτ το σαββατον ου τηρε). This is reason (causal οτ) enough. He violates our rules about the Sabbath and therefore is a Sabbath-breaker as charged when here before (John 5:10; John 5:16; John 5:18). Hence he is not "from God" (παρα θεου). So some.

How can a man that is a sinner do such signs? (Πως δυνατα ανθρωπος αμαρτωλος τοιαυτα σημεια ποιειν;). This was the argument of Nicodemus, himself a Pharisee and one of the Sanhedrin, long ago (John 3:2). It was a conundrum for the Pharisees. No wonder there was "a division" (σχισμα, schism, split, from σχιζω) as in John 7:43; John 10:19.