Starting air explosions may be caused by continuous starting air valve leaking followed by it sticking in open position. An oil film may build up in the starting air pipework due to oil brought from the air compressor. With continuous leaking valve (as seen from the video) hot gasses with unburned fuel will enter the pipe and turn the oil film into a hot incandescent carbon. When high pressure air is put on the pipe line an explosion my happen with resultant high speed high pressure shock wave. An example video is attached below. It is unbelievable how bad these valves are.