What you beleive and what you've experienced really is only a small part of the discussion. First of all you have a black aro, I have yet to see proof that black aros can get DE.
As for the head trauma causing DE, I seen it happen twice on two different occassions in each eye with my silver. There are 3 other people who have testified as to the same in just this thread. So it seems more then likely.
Do you think the wild aros in your example hit there head anything like what your black experienced? I 'm sure there are rare cases, but if the aro cannot recover from the DE, in the wild it will die.