The main reason I ganked lowbies on vanilla WoW in lowbie zones is so that, hopefully, if I gank them enough times and they're in a guild, they will call their guild mates to come kill me. This makes it so me and the lowbies lvl 60 guildies get to experience fun PvP in a zone that no 60s hardly ever PvP in. Having different terrain, maybe buildings to run around or in. Also the more guildies they bring to try to kill/camp me, the more guildies I bring from my PvP guild.
Soon ganking one lowbie over and over for 30mins could turn into a 10v10 epic battle in duskwood or Thousand needles(parachute capes!)
Although when I was an angry kid the main reason used to just make the lowbie mad, really really mad by camping his body and having a friend of mine camp @ the spirit healer in the zone so when he finally rezes there, he dies and takes the durability loss. That's real asshole moves