Its mostly going to come from players who come from raiding guilds. For example, I can run HM LI in 15-20 minutes with my guild, never wipe etc, but when I pug it; it usually is an hour ordeal spent explaining mechanics and trying to teach someone how to play their class. It's not even elitism, its just that people from higher end guilds/groups end up expecting better and for the most part, pugs aren't close to the same quality as a good raiding guild.
There is a degree of bullying that happens in groups with 3 guildies plus one PUG member. I often find that it brings out the worst in people. I'm always cautious about a PUG with that group make-up. Sometimes it goes really well and we clear it really fast, but other times you had better watch out because you will become the scapegoat if something goes wrong regardless of whose fault it is.