Nostalrius Begins - Quality wow vanilla realm (1.12)

Just today, I was adventuring in Mulgore. I entered Palemane Rock as part of the "Sharing the Land" quest. As I wandered deeper into the cave, I cleared the area, and the way ahead looked safe. As soon I ran forward, however, I was phased, everything respawned in an instant, and I was overwhelmed by a whole group. Strangely, running back out of the phased area made them disappear and pulled me out of combat. I managed to find a way around it and continue with the quest. I also encountered another phased portion of the cave, which contained a water barrel. I chose to avoid it, as I couldn't be sure what was on the other side. At some point, during my attempt to escape, I wandered into the phased area that I first encountered, and died. When I came back, I was unable to resurrect until I had reentered the phased area. Overall, it was not a major problem, but it was confusing trying to navigate. Does anybody have any idea what this is?