As someone on said spectrum, I'd say I might be more accident prone than someone who's normal, and potentially more likely to get into trouble in general. For example, while I wasn't seriously injuried by any of it, I've been nearly hit by traffic, fallen off various things, actually been hit by traffic and done quite a few things which would probably be seen as rather 'stupid' (in an 'endangers your health' sense).
Perhaps it's possible that due to various behavioural factors, those on the spectrum might be prone to getting into dangerous situations and paying the price for them more than 'normal' people. Being generally clumsy seems to be a common trait here.
There's also a potential darker side here, in that being on the spectrum might make some more likely to end up in a violent confrontation than someone who isn't. There were quite a few news stories in the past where people with these traits have been assaulted or even killed because they did something that somehow annoyed some random scumbag. Maybe people outside of the autistic spectrum might be more likely to see a situation is trouble and stay out of it.
Those things plus the risk of suicide could all explain this difference in mortality.