There are so many reasons why pvp happens and just as many why it does not.
Last night I flew my 64 priest into Tarren Mill (where Alliance have Southshore). All I wanted to do was go get some long overdue tailoring training in town and leave. As I landed I was aim-shotted by a 70 hunter. The pet started tearing me up and a 70 mage with some pretty amazing looking gear nova'd me to the ground and proceeded to slaughter me quite quickly with a fireball as big as a Desolace kodo !
As I died I both sighed and laughed. People camp things. PVP happens. I enjoy those possibilities.
As more horde turn up being made aware by guildies and friends that Alliance were kicking-ass at TM the balance of power moved and the Alliance died / ran. So guess what... payback... we all proceeded to kick-ass at Southshore. Those flying into SS were 1/2-shotted. I'm sure they were as equally suprised as I was at TM.
In all of this people are coming, going, logging, hearthing, griffing / batting in.
As I die for the 4th time at the hands of Alliance at SS and take several Alliance with me each time I remind myself that I just came to skill up tailoring.
I get my body and head out. In localdefence some 31 mage is begging for help due to a "naughty" 51 (?? to him) Alliance shammy killing him over and over as he is trying to do yeti quests. We roll up on the shammy and kick his ass. He is dumb enough to blow his self rez and dies in under 2 seconds. lol
At every single point in this little venture to SS imagine how people see the game from their perspective, not seeing all the noise of the world around them. The quests being completed. The pvp happening. etc, etc.
All of the above is usually water-cooler chat with friends at work (who also play Wow on both Alliance and Horde) but I thought I'd share just to illustrate how truly immense and immersive the game is.
To your original point I agree that one never really knows what is going to happen when meeting the oppositte faction. I've been waved at then killed. Pointed at and helped. Plain out ganked and camped. Heck, every possible combination of these events too. It's funny how someone kills my little 28 mage and sits on her corpse only to wave politely at me on my 70 shadow priest as I tear their face off with no mercy. As I log back to my 28 mage I am strangely left alone by the ganker...