Yeah, the Neptune/Uranus conjunctions happen about half a dozen times less often than resonant conjuctions with Jupiter and Saturn, but are much closer, so it's not obvious which would be more important. Until you throw in the fact that the resonant effects always happen at the same place in the orbits of Neptune and Uranus. So if Grant is right, what must be happening is that the random kicks by Neptune/Uranus get smoothed over by the circularizing effects of the debris disk, whereas the resonant kicks have a chance to cause a gradually increasing orbital radius even in the presence of the circularization. I'm not really sure what causes that difference, but that's the big deal with resonant interactions. Also, I'm personally a bit surprised that the circularization could be so strong as to act on tony873004's 500 year timescale, but I can't think of any better suggestion than Grant's. It does seem pretty mysterious to me.