Well, I have no idea why they *actually* do it, but my young rats do it, too.
However, I assumed that they learned this behavior from their mother always picking the up in weird places, and that the little girls were trying to make other rats move. For instance, one rat would be in the wheel, and a baby girl would start tugging on her ear so that she could get into the wheel. Sometimes, the babies would do it to each other, and you could see them straining as they tried to pick each other up/pull them around.
Of course, this is probably entirely wrong, and it's a completely subjective interpretation made on the basis of human behavior.
I would say its a maternal thing. I noticed my momma did this to the babies she had, and realized that my adult females pulling their cagemates were doing the same thing. It only lasts a few days, I wonder if coincides with their heat cycle? Has anyone ever seen male rats do this behaviour? I sure haven't.
Btw the squeaking isn't pain but more of a complaint if you listen. "Ugh do you have to do this again?"...whine whine. My 2nd in the ranks rat used to pull around the alpha and she put up with it, but plaintively squeaked the entire time...haha. I found that they were always dragging them up to the "nest" or hammock...I seriously think its an overpowering instinct for nesting/mothering.
I've had Blanche for maybe a month, so looks like it's not going away on its own. Maybe it does coincide with them being in heat?
After she does it, usually Poopsmith will come around and show her who's boss with some heavy grooming, but that doesn't seem to dissuade her from trying again. What's funny is Blanche is the biggest rat and I think she could be boss if she just wanted to.
I would agree that they probably aren't squeaks of pain, though it does look like it must hurt. But i've never seen any scratches on any of my rats so I don't think they are seriously hurting each other.
My girls started doing it at 3 weeks to the 1 year olds... which makes me wonder if it really is a dominance thing. I've never seen a rat higher in rank do it to one lower down. Maybe it's a denial-of-passivity thing.