I think what the game registers as you press "Z" is a light R button press and an A button press. You can see this when holding Z, as you first grab, then the shield goes up.
It could be that it registers it as repeated A button presses, because here's what I noticed about the C-stick: if you hold down R (or something to that effect, don't remember what button it was) and hold c-stick up, you'll jump repeatedly. This means it's registering a c-stick press as repeated smashes of the control stick (or something like that?).
Z could be doing the same thing: registering repeated A presses, so as the beam goes along it grabs anything.
Or not. Wild guess.
It could be that it registers it as repeated A button presses, because here's what I noticed about the C-stick: if you hold down R (or something to that effect, don't remember what button it was) and hold c-stick up, you'll jump repeatedly. This means it's registering a c-stick press as repeated smashes of the control stick (or something like that?).
Z could be doing the same thing: registering repeated A presses, so as the beam goes along it grabs anything.
Or not. Wild guess.