wtf....
Yes, serious failure, and yes, you could have come off a LOT worse than you did...glad you walked away
But talk about an over-reaction from some people
My first thought was the same as JB's....that smashed casing shown in his pics.
There's a full video that goes along with that photo that shows what happened though...and for an off road bike, perhaps it shouldnt have happened, but that was (until now) the only one that ( I think) has been reported here......amongst how many 1200's?
I think its important to keep it in perspective....without very close examination of the whole casing, it is obviously impossible to say what happened.....it COULD have been a casting failure, but given its age (and presumes mileage if you are commuting), I would have thought it would have shown up a lot earlier in the bike's life.
It COULD have been a stone from the French drains alongside the A12 that may have been flicked up from your front wheel, or bounced around the rear wheel, or maybe even got fired across from a car in the next lane to you when their wheels hit it......There are long sections of the A12 that have tonnes and tonnes of sea pebbles in a ditch along the side, and when trucks or cars go into them (very common) you get hundreds of metres of carriageway strewn with shiny, rounded, fist sized pebbles
The UJ is clearly another prime suspect.....riding straight , along a smoothish road at sensible speed, there would be a lot less forces acting on the swing arm that it would have experienced in other situations, but if the UJ did let go, particularly at a high speed (only 70 honest) then it could easily have thrashed around enough to shatter the case.....which is designed to support and withstand stresses from other directions entirely.
In short, glad you made it out of the situation undamaged, no idea what caused it but it's a very very rare thing......the first I've heard of in that sort of situation.