I believe it does have to do with the metal movement of the
flan or
planchet when struck by the dies. The metal movement at the center of the
flan or
planchet is more toward the die cavities (design elements) than towards the outer edge (though there is some movement in that direction as well). The metal movement at the periphery of the flan/planchet is increased towards the outside edge. This is accentuated in
ancient coins because they were not struck with a die collar, so there was nothing to restrict this movement of metal. This greater movement of flan/planchet metal at the periphery caused an increased level of die wear in this
area.
The US
mint used an 'open collar' until the 1830s, when they switched to a 'close collar'. Pronounced flow lines are often seen at the periphery of early US coins, the
bust half being a
good example. After the switch to the 'close collar', there was a great reduction in metal flow die deterioration at the periphery.