I had a GM explain what was the current theory on that: due to heavy server
load it is possible for more than 1 person to fight the same mob, i.e., the
mob gets twinned. When the server recognizes the conflict its means of
resolution is to rejoin the two entities in a "safe" location. Makes sense
on the surface even if he was shooting me a line. :)

And as for the z-axis "feature"... I was the king of all chicken pullers
since all I had to do was stand my troll shaman at the base of the tree and
they would come running immediately after a new flock had come home to
roost! What fun that was to RP every spawn cycle differently. :)