I do think it's seasonal, however not connected to GSoC. Many of the major contributors are (or were) students (Julia seems to be detrimental to people finishing their PhDs, except in the case of Jeff Bezanson, where in some sense he crowdsourced a lot of the research, as Julia was the topic of his thesis ;-) ) Many who aren't students are professors. Definite drop off when people are taking classes, exams, teaching, etc.