It's probably Baader-Meinhof plus an actual increase in frequency, which can happen if people see an interesting link on NH and then share it socially on other sites (which then gets shared further). Interesting topics seem to be fairly "bursty" among various social networks.
Yeah, I've been aware of the demise of the American chestnut tree for years now, but I'm just seeing it in heightened frequency lately. You're right that social networks can get bursty.
