As we know little ponies grows fast and Luna’s old bed is small for her now so he decided to find new, more comfortable bed for herself.  
Unfortunately for Celly who also have to find new bed now, and unlike Luna her new bed will much less comfortable^^
 
Actually Luna should learn manners and understand that this is unacceptable. Today she usurps Celestial bed, what will next, Celestial throne? Such behavior can lead to civil war^^  
But everything can be forgiven if you are a little pony ^_
 
(And just one more little thought from my head)