I wonder whether its because a large load of capacity needs suddenly in unaffected AZs put abnormal stress on things like networking to the whole unaffected AZs.

My only experience there is in Azure, when they deployed patches for the Heartbleed etc issues, for a few weeks things were much slower in CPU power (our response times just shot up 20% for no reason then recovered a few weeks later) and there were network related timeouts that were abnormal and it all settled down eventually.