We spotted it straight away in our house. You saw the marshals panicking and a plume of steam coming from behind the mobile crane. Then Bianchi's name slipped down the ticker at the bottom of the screen until it said 'Bianchi Out'. Then there was the shot of the medic with a stretcher and...
Eyewitness account is that he struck the back of the tractor:
http://www.motorsport.com/f1/news/bianchi-moved-to-icu-following-brain-surgery-as-more-details-regarding-crash-emerge/