A few possibles.
1 The act of catching the pleco disturbed the substrate/decor relasing some trapped food. Your upside down is chasing down all the small pieces of food now floating in the water.
2 The pleco had his terrorory and would defend it against the cat. Now with no pleco the cat can go anywhere and is seeing what else has changed.
3 My upside down fears the net. If there is a net in the water he panics and stays stressed for a while.
4 Your cat may have "schooled" with the pleco and is searching for his buddy.