The US suffered 290,000 combat deaths in WW2. They also suffered 113,000 military deaths marked "Other" adding up to a total of 405,000 total deaths.
What exactly does "Other" mean? What are the causes of death that fall in this category and why weren't they marked as combat deaths? 113,000 dead soldiers isn't something you can just pass off under one miscellaneous category without further explanation in my opinion.
They're deaths that happened outside of combat situations. Deaths from disease, training accidents, car crashes, accidents in rear areas. They weren't counted as combat deaths because they didn't occur as a result of enemy action. For example, if a warship sank in a storm, as happened to USS Hull, the men who died were counted in the "Other" category. Had their ship been torpedoed, bombed or hit a mine, their deaths would have been combat deaths.
There were thousands of training accidents in air training alone that probably accounted for some. This sourced blog post: http://finalflightthebook.com/blog_Jan011.htm gives some perspective. If his quoting is correct, at least 10% of that total was from air accidents in the United States alone from training. How many aircraft were lost without engaging the enemy due to equipment failure?
Basically, that other is everything non-combat related. That is accidental, disease, etc. It seems like a lot, but in reality is way less than previous wars.
To add on the other comments, this also includes suicides, court martials.