Wikipedia's sources suggest it's a metaphor conjured by developers, as you suggested:
> An early instance of this sort of query was when Michael McLay publicly asked, in 1994, what would happen to the Python language if Guido van Rossum were to be hit by a bus.
> "Truck number" was already a recurring concept in the Organizational Patterns [of agile software development] book published in 2004, itself an evolution of the work published in the first book of the Pattern Languages of Program Design series in 1995, which was the publication record of the first Pattern Languages of Programs conference in August 1994, where it was referenced in patterns including Solo Virtuoso. ... It was seen in engineering by 2003, and the Debian project in 2005.
It is a lot less neutral to say an employee was "fired" or "quit"
For anyone who hasn't led a wonderfully sheltered life that lets them conceptualize the situation as if it were something out of a "Looney Tunes" cartoon, it's going to evoke imagery of pools of blood and viscera on asphalt.
Not the kind of mentality you'd usually expect from an HR department.