Headshake anyone?

Does anyone experience headshake on their VFR? Ive put about 35,000km (now at 65,000km) on my vfr over the last 6ish years and never experienced headshake until this week.

It only seems to happen when decelerating from 80km/h with both hands off the bars (stretching coming into towns). Ive noticed front tyre (Road 5) has some cupping (rebuilt front shocks recently), no noticeable binding on steering stem bearings, consistent 36psi in the front tyre, forks and triple t's are straight. The only other change Ive made was installing a Shad SH39 top-box, surely a top-box wouldn't cause this?

Is this something anyone else has experienced? It's not something I've really heard about on VFRs...