Assuming the audience is relatively unchanged, peaks and troughs in email responses are typically attributed to the offer/content and the subject line.
And, naturally, we take that into account when planning future content/offers and writing future subject lines.
But, of course, a proper analysis should consider all the factors that might impact results. And there are a lot more than we generally think.
So when you come to assess your results, cast a wider net when looking for an explanation. For example, review delivery issues, copywriting approaches, timing, the previous emails, your other marketing, spam and the competition.
Read this very interesting article by Mark Brownlow here: www.smartinsights.com