I guess my suspicion is that the big contributor is auto accidents, which had no analog in 1900 (and in any case, transportation accidents at muscle-powered speeds were unlikely to be fatal). Still, my intuition had been that the world used to be a more dangerous place than it was.
Then, as now, the most common accidental cause of debilitating injury and death is quite humble - trips and falls. As our population ages, this problem becomes more and more serious due to what geriatric specialists call the cycle of falls.
The cycle starts when an elderly person has a minor fall, which causes them to be hospitalised or bedridden for a few days. This period of inactivity causes muscle atrophy and a decline in coordination, which greatly increases their risk of falling and their risk of being seriously injured when they do fall. Without appropriate intervention, a fit and healthy older person can become extremely frail within the space of two or three years due to a series of increasingly more frequent and serious falls.
Actually there was an article the other day about accidental death rates: death due to drug overdose has overtaken death due to car accident, due to a dramatic drop in one and rise in the other.
http://abcnews.go.com/Health/Drugs/drug-deaths-exceed-traffi...
The extent to which drug overdoses can be called "accidental" is, of course, debatable, but I guess they're classified as accidental at least in cases where it's not obviously suicide.
Autos and airplanes just aren't comparable.