[Doug I]

I'm with Buzz - the flare at the toe of the spinner ear appears to have a large enough radius that it's able to force the safety pin down by wedge action. Doing it by hand may not replicate the result as all the vibration etc from road conditions are missing.

I'd still put it down as 'freaky' that it happens. So many variables have to be 'just so' for it to happen.

Safety wire is a misnomer. It will not prevent a nut/bolt/spinner from working loose - wired properly it will however show evidence that something has moved. Installing safety wire correctly is an art.