Hive died off in one week...

[Matz]

Just wondering if anyone could explain what happened.  I'm a first year BeeKeeper in Canada and started with 2 hives of Cornolia's.  One of the hives has been really strong right form the get go, but the other one has been struggling all summer.  As i was getting ready to start feeding them for winter last week, I took 2 brood frames from the really strong hive (had 3 deeps of brood) and placed in the weaker one hoping that it would help.  As I was rearranging the weak hive, I found 2 superceeding queen cells and 2 swarming queen cells all were already empty and could not locate the origonal queen.  After placing the lid back on the hives and placing my feed containers on the tops of the hives. I noticed that there were numerous black bees (not my cornolia's) gathering on the weaker hive and taking any dripped syrup as well as fighting with the bees on the outside of the super and entrance.  Now less than a weak later, my weak hive is completely dead, there thousands of dead cornolia's and black bees everywhere including inside the hive, the frames have all been robbed and the foundations are being demolished.  I realize that its too late and there is probably nothing I can do now, but is it normal for other strains off bees to raid and kill off entire hives just like that, in less than a week and any ideas on how to prevent that in the future???  Any info or help would be more than appreciated, "Thanks" in advance...  BTW this site and all the info people place on it is great!!!