That would mean you have your Party on Round Robin. To turn it off, you must be the party leader. Right click your character icon (where you name/hp/mp/etc is), and click "Change Loot Settings". Set it then to "Free For Fall", then click "OK". It should then make it so you don't pick up any drops from your pals in party!
An another method would be just leaving party and rejoining up again, but I think with many people in a single party, the first option would be easiest.
Happy Gaming. o/
Current Obsession: Overwatch