One reason technical ebook sales might be front-loaded is that this makes sound economic sense for the purchaser.
Suppose you hear about a new book on Advanced Widget Techniques. You aren’t really building widgets now, but you’re planning to put a widget into your product this fall. You don’t expect to need any advanced techniques. You might be hiring a widget consultant to do the work in any case.
So, you definitely don’t need that new book yet, and you might not need it at all.
BUT, suppose it turns out that you do need it. Come autumn, you install your widget and the compiler starts talking about Error 1708, and suddenly you need those advanced techniques. But how long will it take you to remember that book, find it, and get it delivered? You could waste an hour tracking it down, and that’s a lot more expensive that buying the book. Your consultant might waste an hour tracking it down, and that’s a billable hour. And the consultant’s going to expense the book anyway.
So, buying the book before you need it might make sense, even if it’s likely you won’t need it at all.