It occours in some situations, not quite reproducable.
Example; One hearthling (position A) tries to grab wood from a fallen tree (position B), moves some meters, then gets a new task by AI (get wood from storage position C, aborts his/her queue) and moves to point C to get to his wood-pile while exiting his path to pos B.
Now he/she grabds wood from pos C, moves back to pos B, before moving to pos A. 
I did modding for some games and we had ACTION_INSERTMODES and we had similar problems in our first steps sending those stupid ambulances to their right positions via “AI”.
I do not know how you call it, but I suspect you are constantly looking for closer wood (for my example) and do a ACTION_INSERT with saving the old action, going to position B.
So, first AI action: get wood from fallen tree
ACTION_NEWLIST - get wood from fallen tree (position B)
ACTION_APPEND - get back to workplace (position A)
ACTION_APPEND - do your action
… finding a closer wood -> ACTION_INSERT, which accidently does not kill the first task:
ACTION_INSERT - get wood from storage (position C)
ACTION_APPEND - (old NEWLIST/FIRSTOFLIST) get wood from fallen tree (pos B)
ACTION_APPEND - etc… back to workplace/do your action
That should explain why they are sometimes moving back and forth…
…hope my sloppy english is understandable… 
Alpha 9 .240 x64