Having a number of the same type of villager in close proximity causes them to link randomly to other workstations. 

On our realm we have a couple of areas where villagers have been employed as say librarians then locked into pods with the workstation in front of them. They have all been traded with up to max level. Almost daily they un-link from the workstation in front of them and re-link to another workstation that might be 20 blocks away in front of another librarian. 

This is not specific to librarians as it has been noticed on Clerics, Stone Masons etc. I have also seen a fishermen who has been traded with suddenly turn into a farmer then back again. The fishermen's trades also change every time he re-links with the barrel. 

This doesn't seem to be related to specific locations as we have experienced it in 2 separate trading halls which are 300 blocks apart.