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'Unstable' is perhaps a bit of an overstatement, although it conveys a good impression of what's happening.
I don't have a detailed hypothesis, but consider the following:
First, the same thing happens with loads lower than the optimum - accuracy, as measured by group size diminishes, although not at the same rate.
Second, the optimum load can and does vary with each individual gun, although within 5 grain increments many will have the same optimum load. I believe this is due to barrel dynamics, which are very unpredictable and can vary greatly between examples of the same design and manufacture.
Third, there are technique issues that can affect the optimum load results, so that what works one time won't work the next time because the technique was changed - generally, this is the amount of compression used.
Notwithstanding all of the above, I think the most important characteristic that causes accuracy to decrease at maximum load is rifling; I believe the projectile is pushed faster than the rifling can accept. Other variables, such as the projectile alloy, depth and shape of the rifling, etc. affect this so it's not something that a broad generalization can explain, however.
And no, I know of no test data aimed at explaining the phenomenon, although I've done lots of my own testing confirming it exists.
Anyway, that's the best I can do on one cup of coffee while also attempting to keep the livestock from destroying the homestead while SWMBO recovers from a cold. Perhaps a dedicated thread on the subject would be interesting some day.
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