It looks like the LR suspension broke under the extreme braking forces incurred at the chincane. As he's starting to spin you can see that the LR wheel assembly is deflected outward by several degrees.
The rules are very clear and every driver is required to know them and adhere to them. The driver is allowed to place two wheels over the line, not four. The resulting penalty, had there been time enough during the race to fully review and assess the incident, would have been a drive through, but lacking the time they instead chose to assessed the penalty as an equivalent loss of time after the race, which is well within the rights of the FIA to do so. If Vettle, or anyone for that matter, had cut a chicane in order to complete a pass it would have been exactly the same. The stewards would have reviewed the incident, given him an opportunity to correct the situation by giving the position back, or if he was too far ahead to pragmatically do so at the time he would have been issued a drive though penalty during the race. If Vettle couldn't get the pass completed and stay within the boundaries of the racing surface he should have backed out and tucked back in behind Button and tried to pass him again either later during the lap or the next. As one of the world's 24 best drivers he should have known better and drove accordingly.