I thought I knew this, but I guess I forgot.
From what I've seen it's not total views but instead # of views/arbitrary time period, say views per hour over the last 24 hours. If that ratio exceeds a certain threshold then it becomes popular. That's the most basic way to calculate it but I suspect that the threshold moves around based on some sort of average within each category meaning timing and luck plays into a new guide becoming popular. I would imagine that an Instructable could go popular years after being published if it started getting enough views in a short enough time.
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"Popular" is also triggered by frequency of comments.
One would imagine, more than the n'th place views in the last x time period.*n = the cutoff of what is posted as popularx = timeframe cutoff, week, month, whatever.