What was the biker doing? Just as well the driver was alert!

I would say the biker was trying to catch up with the two other bikers that are shown earlier on in the video traveling away from that island.

I think the riders were seperate... Previous 2 were Tourey Adventure Types... Looks like the are used to riding together going by their off-set road positioning...

Mr Gixer came a cropper on ya classic 'Roundabout Granules'... (very similar to those found at keep left signs and trafic lights) together with still approaching on the brakes... was wide lined and foot down when he came in to view...
Plus a side order of 'not looking through the bend' and a pint of 'target fixation'...
Giving himself a bit more room, Better Forward Obs, - Ready to clear the car and van in a one-er before they catch up to the lorry... and a Slow In / Fast Out mindset would of kept his Rizla of a jacket from getting torn...