Oque server changed when i added group

Quick question becasue wtf...

I found one of the rares on my server (Gorok) but since ive allready got the mount i decided to announce in Oque to be nice :)

so i was stood nexto it and next think i know i invite people and the mob dissapears infront of my eyes... :s it changed my server??? wtf why>? how do i avoid this in future if i want to announce for people???