While it's hard to account for this in exact numbers, this selection is surely happening.
This also interacts with the cause of death: Fighting late stage cancer makes it hard to attend. And while on premises it's much less likely to get into a fatal car accident. Plus Burning Man doesn't seem to be the place you're going to be mugged on the way to your home/tent. The car accident factor alone would lead me to expect that most fatalities associated with Burning Man happen while getting there or on the return trip.
The cause of death might be scrubbed from the statistics to get a more realistic (read: smaller) number.