Learn English – Meaning of “out of an interest”

I found this reference in The American Heritage Dictionary of Idioms (1997) by Christine Ammer:

leopard cannot change its spots, a Also the tiger cannot change its stripes. One can't change one's essential nature. For example, He's a conservative, no matter what he says; the leopard cannot change its spots. These metaphoric expressions both originated in an ancient Greek proverb that appears in the Bible (Jeremiah 13:23): “Can the Ethiopian change his skin, or the leopard his spots?” It was first recorded in English in 1546.

The wording of this entry remains unchanged in the second edition of Ammer's dictionary (2013).

In the case of Mubarak supporters, the writer is implying that—more than changing sides—they're changing (or appearing to change) their very natures. Having once supported the man and his regime, they're now adopting a different coloration as a way to protect themselves in a society that's still reverberating with revolutionary shockwaves. The more common negative usage—that animals (and presumably humans) cannot change their stripes any more than their essential natures—is slightly more common than the positive usage employed here, but both meanings are widely understood.

The latter: it's not a standard word that you'll find in any dictionary. The suffix -athon, as you've guessed, comes from marathon, and means "an event, as a sale or contest, drawn out to unusual length, often until a prearranged goal, as the contribution of a certain amount of money, is reached" (according to Dictionary.com).