Given that the pressure in the tank of a 140L at full throttle reaches considereably higher than atmospheric pressure I doubt very much that the changes in atmospheric pressure that you talk of would make any difference due to the pressure in the tank.

The humidity and pressure would affect the required mixture setting though, so if the engines were tuned in much lower humidity and higher pressure in the days before the competition then come the competition they would be slightly off-tune. It could be that the DZ's are tuned slightly richer on a day to day basis and can therefore cope better with the tuning changes required by your extreme weather.

Apart from that I cannot think of anything that could cause only the L's to die and no other engine, other than a coincidence (which does happen!!)

James