Apologies for using my own quote but could someone explain why the Vampire "Elephant Ears" being now placed beneath the fuselage instead of on top, would cause the aircraft to pitch up quite strongly at critical mach without any apparent action on the part of the pilot? It was never explained to us at the time. Mind you, memories fade somewhat after sixty years..
Purely speculation here ...
The characteristics described for the "ears on top" scenario sounds a lot like the tailplane being blanked by a large amount of separated flow off the upper fuselage, that separateion being triggered by the ears location at a position where the cross-sectional area was high and thus susceptible to early formation of shocks. So rather than a direct effect due to changing the lift distribution on the fuselage, it's a secondary effect through action on the flow around the tailplane. (If it was a change to the fuselage lift distribution, you might have expected a pitch UP, as described in Keith's paras 2 through 4 for the first shock on the upper surface of a wing (with the fuselage considered a crude aerofoil).
I'm going to speculate that the ears-under configuration still had a shock forming, but this time of course with separation below the fuselage, and perhaps this iduced a bit of additional downwash on the tail (the upper surface flow maybe being 'pulled' down into the low presuure in the seperated flow?) If so, that additional downwash would cause a pitch up.
Pure speculation - it'd take access to test data to provide the correct explanation, assuming it ever existed...