Now I remember you mentioning the biplane's incidence angles. I think the stall initiating the flick could have to do with that, but it would still require that it's an inverted stall (that is the airplane is borne by its wings and not by centrifugal force). Inverted, the upper wing is now lower and the lower wing is upper. The upper wing stalls first and now it is the back wing, meaning the remaining lift is more foreward (in front of the C/G) what makes the stall self-reinforcing so the slightest onset will cause it.

P.S.: Only later I remembered that you mentioned your D7 not stalling like the Nieuport. That's tough because their configuration is very similar. Could just be a different airfoil, but I would rather assume that the Nieuport's elevator power is scarce, the reason why still to find out. It seems to need full up elevator just to keep the loop's upper part round or it would climb up in a straight line until it runs out of speed. (I would try that out.)