Loose front sprocket ?

I'm a at loss trying to figure out how my front sprocket nut has worked it's way loose with the locking washer still bent over

I fitted it myself as per the book so i know it was done right,i dont thrash the nut's out my bike's and am fairly good on the throttle input's,i do pull the occasional wheelie but would this really cause a loose sprocket nut ?
Any thought's welcome


Click on photo for small vid

Havin watched you ride i no the only loose nut is in your head mate

How do you wheelie Ginge, off the throtle or do you clutch it up? I only ask as in theory, clutching it up could eventually loosen the nut!?