I think that most of the posters here would offer that it's never only speculation that drives these books. Usually the trigger is quality, which starts the cycle. What suggests speculation is sudden spikes in price, especially early in a book's life. That's not consistent with accumulation of readership, which usually happens over a longer term.
My opinion is that this is mostly speculation, but, as with all my opinions, I may well be wrong. I'm used to arguing stock market positions, where being proven wrong, or just flat out pantsed, is par for the course, even with a well thought out thesis.