It's not just SEO, it is practically any subject on HN that gets a periodic wave of interest.<p>I think it is simply tied in to the nature of the web. If you 'inject' a bunch of users via a link on HN in to a spot on the web then from there they will spread like sinking a mineshaft will exhaust the resources in the neighbourhood of the shaft.<p>Then, when that particular mine is exhausted (no more interesting content to be found around that locus in the web) the subject dies down until someone finds a new and untapped reservoir.<p>I've seen this happen around all kinds of subjects, from programming languages to hardware hacks and political stuff.