Left (main) hand with bow spins when holding shield in offhand and you left-click and instantly right-click after.

Left (main) hand with bow spins when holding shield and you left-click (hit) and instantly right-click (aim bow) after.

https://youtu.be/FCp_6f7J6iE

It also happens with no arrows in inventory.

It does not happen when you have [Main Hand: Right].

It does not happen with other items in offhand.

It does not happen when the hitting animation has finished already before you start right clicking the bow.