When someone leaves a project quickly and unexpectedly. Typically referring to the risk associated with the tribal knowledge someone has that puts a project or business at risk when they leave the company. Some one would leave if they got hit by a bus (Die) or win the lottery (Not need the Job anymore)
Sue won the lottery bus and will not be back to work. (Not knowing why she is gone)
If John gets hit by the lottery bus we will all be hurting to get this project completed on schedule.