From what I've read, the overwhelming leading causes of death and injury in the outdoors are falls and drowning. This makes a lot of sense, since people will engage in riskier behaviors such as scrambling up steep slopes/rock or crossing streams despite not always being very experienced in either activity. In many cases, the consequences of mishaps carry with it far greater risk than originally perceived - thus one could blame poor judgement on these accidents.
Heart attacks account for a fair share as do lightning strikes. Bee stings cause far more deaths than animal attacks, which are incredibly rare.
Mountain House Chili Mac also inflicts a large number of casualties every year.