My Internet connection went down just as I was replying last night...glad it did because I remembered a statement from one of my references this morning:

A demagogue can only succeed in an atmosphere of free speech and free expression, since … demagoguery is intimately bound up with rhetoric

I'm not sure that's true, but what it does address is that some environments are more receptive to demagoguery. I would guess (no study to cite) that a discontented population is one of those environments. When people are unhappy they look for extreme solutions, and scapegoats. Demagogues offer that. The level of discontentment rises and falls over time, and so perhaps the "waves" of demagoguery. Of course, this is just conjecture :)