Psychologists have studied the loose spoon phenomenon, and it mostly comes down to the human desire to get as much quality sleep as possible. Once relationships progress from the honeymoon phase, priorities tend to change and being all loved-up is no longer a good enough substitute for catching a full night of Z's.
In the regular spoon position, the big spoon communicates "I'm protective of you." In the loose spoon position, they're saying "I'm still here to protect you and you can count on me, but I don't need to smother you." This level of comfort in a relationship is healthy and psychologists say it's a good sign if you have a long-term partnership and still favor this sleeping position. You still like touching each other, but you don't have to be all wrapped up.