I used to build shit so I know how some of the physics work, and the answer is none. The hill isn't steep enough for the one bolder to gain enough speed to Hit the thing with spikes hard enough to cause a chain reaction so no chain reaction will happen unless if there's 20-100 mph winds pushing the first bolder or some unexpected factor coming into play to cause an unplanned event to occur.
And in reality D would most likely crouch to evade the bolder so he'll be fine, he has enough room to do so.