This is his prey drive acting up. Just like he would for a squirrel or chipmunk, its just another small animal and all fair game to him. Work on teaching the command ‘leave it’. It can come in handy in many situations but especially ones like this. Also helps to see the cat first and anticipate his reaction. When he spots it give the command firm and clear. Also I would suggest bringing a toy or some treats with you so you can praise him with it when he does. That way you are redirecting him telling him to pay no attention to the cat, then he looks to you and is rewarded for it!

sounds like he is just curious.