Ok, the take form the folks at While Labs essentially is that because I pitched at such a low temperature and held it there through fermentation until clean-up time, this condition may have stressed the yeast sufficiently to cause the production of Isomyl Acetate, which is the banana aroma I'm getting. Still doesn't explain why it dropped in 4 days, but there it is.
Next time maybe I'll pitch at 62 F, and once active, I'll drop it slowly to 60-ish and let it rip until done, then raise back up to finish out.