Well to call a shooting "random" really depends on citcumstances of the shooting. If the killer(s) target a person who they know or know well then its not ramdom because the person who was killed, was killed for a specific reason. Such things have happened in school shootings and when workers have gone "postal" because the victim or victims is someone who is hated by killer or the person(s) had done something to the killer in their past which warrented revenge or someother course of action in the mind of the killer.
For a shooting to be considered random the killer and the victim normally do not know each other or if they do know each other then they don't know each other very well. A good example of a random shooting was last night in the local news a cyclist was shot for what appers to be no reason so until the evidence shows otherwise it will probably be considered a random shooting by the police who are investigating.
Link to the article:
http://www.wpbf.com/news/south-flori...z/-/index.html
Edit: I just did a quick bit of reasearch on the first person who went "postal", which was Patrick Sherrill a USPS worker who worked at a post office in Edmond, Oklahoma. The proposed movites for his actions where " On the afternoon of Aug. 19, 1986, Sherrill was reprimanded by two supervisors in a glassed-in office over personal matters unrelated to the profession. This coupled with the fact that he thought he was he was going to be fired could be possible motives." The two supervisors who reprimanded Sherrill where killed becuase they where talking about personal and private matters relating to Sherrill and the fact that Sherrill thought he was going to be fired. So two of the fourteen victims he killed he a reason to kill them, the other 12 who where killed where killed for no reason and could be considered shot at random.
http://en.wikipedia.org/wiki/Patrick_Sherrill