I'd have some trouble believing that this was a learned conceit, not unlike the s in English island or the b in doubt, so perhaps it is dissimilation.

It's a good question. It's either--as you suggest--dissimilation or some kind of learned retro-etymological restructuring. My library is in upheaval (shelves moved and such), but I looked quickly in Menéndez Pidal's Orígenes del Español (which I did find), but didn't see anything apropos. I'll ask around.

Ceci n'est pas un seing.