The book I have, Exodus To Arthur, by Mike Baillie has a chart that shows a downward curve that begins before 540AD. 540AD is just the bottom of the curve. Something was going on prior to that date. Maybe a yearly pounding of Tunguska type events for a number of years before the earth moved out of the comet dust/boulder path?
No trace of that around 540AD in the The Anglo-Saxon Chronicle http://sunsite.berkeley.edu/OMACL/Anglo/part1.html