You think so? I dunno, it seems the only people his clique have bullied to such an extreme as to invite revenge were Nagare and Yuda, so it would be natural for Okaya to suspect Nagare's parents by exclusion, since he's been keeping an eye on Yuda. I see no stretches of imagination in his reasoning.
If you what you mean is that Okaya is suspecting a third party out of the blue… hmm, yeah, it does seem flimsy. I mean, all he has to go by is that Kowase, Shikimi and Kinugawa's disappearances were expertly done and Iijima's was crude. But I've seen way worse in terms of leaps in reasoning.
If anything, the two random women talking about Nagare senior's incident in the middle of the street while Okaya is passing by is the more eyebrow-raising event in this chapter. Way too convenient.