The cause of the fire is unclear, however the poster of these images on R8Talk.com speculates the following,
"The only explanation is that the guys who took the car for a test ride spilled some fuel. This fuel went down the down pipe and spilled over the hot exhaust when they started their test drive."
According to AutoBlog, the Audi pictured was a test mule, and not a production model R8.
Unfortunately this is not the first occurrence of an Audi R8 test mule bursting into flames.