Picard is retired from active duty. Although he is still granted the respect and most of the privileges due to an officer of the rank of full Admiral, he has no actual authority. As Riker was made acting captain of the Titan he has the authority to order anyone off the bridge if they are interfering with the operation of the ship. There have even been times in other Star Trek series where a captain was able to have an active duty admiral removed from their bridge because that admiral was endangering the safety of the ship.
Captain Riker says to Picard, "remove yourself from the bridge." This means he's given Picard an order. However, Picard is an admiral: an admiral outranks a captain and a captain cannot order an admiral to do anything.
However, Riker has been officially placed in command of the ship, which gives him absolute authority on the bridge, even over a retired admiral.
However, Riker has been officially placed in command of the ship, which gives him absolute authority on the bridge, even over a retired admiral.
In the flashback scene, Troi calls Riker at the bar and asks him to bring the whiskey he's drinking with Picard. However, when he leaves he doesn't take the bottle of whiskey.