Cyanobacteria, other microorganisms and cyanotoxins abound in the soil and the air after it rains in the desert. Usually there are no people around when this happens. There are 70,000 people concentrated in one spot and many have roughed up lungs from smoking marijuana. Some of these people will be very sick by the time they get home.
@ 93 - that is a rationale hypothesis
This is the most likely explanation.