It happens when ...
melting slush in the otherwise empty centre tank collapses and slips down into the fuel scavenge intake areas - both at about the same time.
The fuel scavenge pumps (powered by pressure from the main tank pumps) then start to transfer the muck to the main tanks. With one wing slightly higher than the other (was the cross wind from the South on that day?) this purged muck slides down one main tank faster than the other, so the right engine gets it first, followed a few seconds later by the left.
Only an idea, but as a jigsaw puzzle piece it looks to me like a better than average fit.
But what do I know? I only fly singles with gravity feed.