The biggest factor is selection bias. If you are in the hospital with some terrible medical condition you aren’t going to Burning Man.
It would be interesting to look at cause of death at Burning Man, I would expect it to differ significantly from the population overall (even accounting for age).
> The biggest factor is selection bias.
This is, in fact, stated rate at the top.
There might not be data to support it since it touches on legally sensitive subjects, but the deaths are almost entirely attributed to drug use.
Deaths due to overdose at music festivals in general are not uncommon and are kept quiet (somehow).
Given that firearms are prohibited and motor vehicle traffic is limited to 5mph, that alone would take out two unnatural causes of death. Also, drowning is a lot less likely to occur in the desert.