Apparently, they aren't the only butterflies suffering (and we must include moths). One guess from the Monarch Watch blog is the incredibly wet and stormy May / June much of the central part of the country experienced. In any case, the blog states that their best guess is that there will be less egg laying and butterflies in late July / early August; this last batch of monarchs is the one that makes it to Mexico and overwinters before heading north again in spring.
Of course, being a conspiracy theorist and fan of any canary in the coal mine hypothesis, I think other issues could be involved. But then again, what the heck do I know--I ain't nobody nohow. I might go check out Robert Michael Pyle's logs on Orion to see if he's noticed anything on the general ebb and flow of North American butterflies. (Pyle is traveling NA for a year in an attempt to i.d. as many butterflies as possible.)