How long was the papers in the car and was they stacked up. This is my call
Being a FIre Fighter u see it all the time and if it was hot out side, you moved the enough to get a little more o2 it will happen
Working in the paint business, I have heard of stories where rags that were soaked in solvent caught fire. Just a possible cause. The most important thing though is that you are safe, and you were there when it happened.