human activity user changing

Hi everyone, i have a process with a subprocess in the subprocess one task is magicaly changing users.

Example yesterday a user had 4 task that wasmt supose to be in is taskbox and today the task have gone to another user.

Is there any sugestion on how to track and identify the causes for this behaviour?
Are you sure that another user didn't click the task in his/her taskbox? Because when you click it, the task is assigned to you, which means it won't show up anymore in other taskboxes.