Sorcha, it is from observing my two grandkids. At first I thought It might be a North/South alignment but when we moved the bed to another wall they still insisted on sleeping crosswise.

From Petr's post above, there is a possibility that it might be some kind of cuckoo instinct whereby they kick weaker siblings out of the bed; at the same time as positioning themselves where they are more stable and less likely to be kicked out themselves. And all the while maintaining access to their food source.