I believe that ‘Effective Cycling’ would dictate that you should have been maybe further left in that lane signalling your intention to turn left while allowing any other traffic to proceed to your right in this turning option lane? Being in the middle probably telegraphed that you were going straight to anyone behind you in this particular case and lane situation. Just speculating, but in this case being in the middle wasn’t left enough to signal your intention and the other cyclist assumed he made a clean pass and didn’t try to sneak past on your right anticipating you would go straight and move to the right side of the road once you cleared the intersection.
“When approaching an intersection, position yourself with respect to your destination direction—on the right near the curb if you want to turn right, on the left near the center line if you want to turn left, and between those positions if you want to go straight.” John Forester, ‘Effective Cycling’, pp.246, 1993