When you place something like a sapling or any block in your offhand and place it on the ground, to other players your main hand is moving instead of your offhand.

If you put a block or anything you can place in the Minecraft world in your offhand(it doesn't matter if your main hand is left or right, the bug it with both) and then place it on the ground, your main hand will move as if it is being used to place the block down.