Thanks both.
I think descent might have been exactly coincident or very slightly after the calculated ToD, so it could well have been the jet chasing to get down on the path. With a VNAV descent in HDG SEL your VSD shows nothing apart from the vertical flight path vector so I couldn’t tell what profile it was aiming for….thinking over 4000fpm was a bit extreme I intervened with an average RoD which would have worked but the high RoD may well have been the jet momentarily chasing down to a shallower profile.
Learning point is probably to ensure you request descent well before the calculated ToD in order to avoid barrelling down at ToD.
Airbus treats this very differently…as soon as you pull heading, the jet just leaves you to your own devices and kicks you out of managed descent.