Got instance cd being in dalaran

Yesterday i got a raid in icc 25 at marrow. Raid was weak, and also some guy ninja pulled marrowgar, so i simply left that raid and used hearthstone to not to put instance on a cd and to find some better raid.
Then couple minutes later after i was already in dalaran in another raid with other guys it said "You now own the copy of that instance" and id appeared (old raid probably killed marrowgar)
But how did i get a cd being in dalaran and if i wasnt already in that raid? How does it work? And how should i avoid such situations in future?

Its because you left the raid during the fight, doesnt matter if you left and used hearthstone as long as you were there when fight started you will get ID if boss is killed