“That’s just physical. Every assembly, especially screwed connections with different materials will loosen through time, in particular as the different materials have their own thermal expansion or as an assembly won’t work in the main way it was designed for (don’t underestimate the effect of gravity when you display your figures). So sometimes there’s no need to play with them, some will just loosen up. Our bots live in some kind of way.”

From 

PAPY OPTI on TFW2005
http://www.tfw2005.com/boards/transformers-toy-discussion/373385-uk-toy-discussion-7539.html#post13462808