When the traffic starts to impair neighboring sites, something has to be done. Just about any ISP will do the same thing: block the site with the surge, that could possibly make other arrangements, rather than inconvenience other customers whose traffic is as expected/usual.
The detail missing so far is why Pair noticed today, if it was the same level of traffic as before, or a slow build. Was a new threshold crossed? (Did someone's HN-focused tool go haywire?)
The Pair message suggests end-of-day logs will be the way to tell for sure.