Talking the opening scene where they fight the renegade Borg on the outpost. Something all of a sudden causes Data to be angry. I take the hint to be the guy who says every biological organism. As though he's holding something to lure Data.
During the rest of Descent, we learn that
Lore has set himself up as leader of these Borg, who became disconnected from the collective due to Hugh (from the previous episode, I, Borg).
He is able to
give Data the ability to experience some emotions by sending a carrier wave that affects Data's positronic matrix.
LAFORGE: Data, listen. Lore is controlling you. He's transmitting a carrier wave which is affecting your positronic matrix.
As Troi points out earlier,
only seems to allow Data to experience anger and hatred.
TROI: Data, all I'm sensing from you is anger and hatred. Have you felt any other emotions?
DATA: There are no other emotions.
We later see him adjusting this while in conversation with Data, and the effects it has.