I think it's a straight demand and supply thing. As has been pointed out Aero's are not selling too well in the USA and so that will hit residuals. If dealers have new stock not moving them the consequences for residuals follows.
Phil,
As I am sure you know, as a rule of thumb most cars are expected to loose about 40% of their value in the first two years and there is not really any reason why Morgan's are exempt from this. Currently, the Aero's are running ahead of the depreciation curve and if you want to suffer really massive depreciation on a car in the UK, buy virtually any new Aston.

It will be interesting to see how the new Maserati Granturismo fares, as in the past they were particularly bad though the QP has rescued their reputation to some extent. I may end up keeping mine for a bit longer than my usual tenure!
Basically, and as we all know, most new cars are not investments but should be bought purely for the pleasure that they bring the user.