If you run with a lot of "identity fuzzers" (browsing through Tor, JavaScript off, cookies banned), Cloudflare can't build its trust heuristics and needs to challenge-response more often. I suspect there's overlap between HN readers and use of those sorts of tools, so I think there is a disproportionate number of people around here who run into this issue (whereas most "regular" folk almost never see a Cloudflare challenge / response).