I always thought just the opposite. I thought that getting hit by a bus would be so rare that it would be almost humorous, so as not to worry someone.
If you would say, “What if you had a heart attack tonight?” then someone might think about how poor their diet has been lately and turn a light hearted comment into a worry about death…
Or bus conflict, for us IC designers.
I’ve heard bus more than anything. I’ve never heard of animal related deaths. The trouble with retirement of lottery is that you’d expect the guy to stick around long enough to clean up loose ends. Getting hit by a bus doesn’t imply any wrongdoing or irresponsibility on the hittee.
When I left Bell Labs during the trivestiture, about a third of our center took the package and left all at once. We had about a week from when we had to decide just before New Years, to when we had to go. It was like whole management chains getting hit by a bus at once.
And why are people always getting thrown under a bus? 
Even more complex, the phrase is “throw ______ under THE bus”. Which bus??