I noticed the other day my Shammy got the surgeons needle without a loot roll and the same thing happened again later. Also just now the Needle encrusted scorpion went to a rogue without a loot roll. Is this a mechanic introduced to stop ninjas or is it a bug?
No, because you should still be able to greed roll. It doesn't happen on all items, most items you can roll on but others just go straight in your bag.
Both items are unique. IIRC you don't get the roll window at all if you have the item already. Maybe it was rare case where all other 4 players had the items already, so there were no rolls, just automated loot distribution to only player who could receive the items.