I suspect most people commenting have probably never been to The Tour. There is no single 'truck' that comes through. There are many support vehicles, advertising vehicles, cars carrying dignitaries, police cars, police motor bikes and so on for absolutely ages before the cyclists arrive. If you haven't watched before, it's impossible to know when to expect the race to pass you. If you are standing round a bend or beyond a dip, there would be no clue that the cyclists are imminent. They just appear. There may be marshalls at points where the roads are closed, and the odd one elsewhere, but overall there is minimal crowd control other than at the start and finish each day.
The other thing is the phenomenal speed they travel at; it's impossible to grasp that unless you are there, and it's so unexpected. One minute there's nothing, then this enormous group pass you briefly, then they are gone in a flash.
The person made a mistake in the excitement of the moment, they clearly didn't mean to cause a crash, they just did something thoughtless with an entirely unexpected (by them) outcome. Yes they shouldnt have done it, but I can easily see how it might have come about.