Only the fanfiction forum seems to show this trend, which makes unlikely the possibility that PokéCommunity and Serebii share origins. Also, they don't appear to be exceptionally closely affiliated, or otherwise I would have discovered this forum very early, stumbling around. It's possible that expending to here is or was a trend started by a popular member or group of members, though I haven't ever heard of it. I do notice (unless my evidence is insufficient) that several SPPF members have joined here around the later end of 2007, which could support my theory. I really don't know.
These are the things we are compelled to with the boredom of the intellect.