I will reply from the point of view of the manager assuming good intentions. They either know what you currently working on and believe that the newer item is more important, that's why they throw you around or they aren't aware of what you are currently working on and just reacting to a urgent ask from seniors. You can inform them of what you are doing currently why is it important and then let them decide which item is higher priority. If the priority of the newer item is truly higher you changing context is worth it.