When magnification is turned on, the positions of the Dock icons on screen don't actually correspond to where you need to move the mouse to click them, as they scroll to meet the mouse faster than the mouse moves (if that makes sense). This means that when you touch one of the items the cursor instantly moves to the touched location but the Dock icons suddenly jump past where they are expected to be (due to the magnification function). To overcome this to need to turn Dock magnification off.
Technical note: We could probably devise a hacky method of fixing this issue by determining if a touch is in the dock and over what icon and calculating where that icon will actually be on screen when magnified and moving the mouse (touch) to the appropriate position but given there is a solution by disabling magnification we are not sure this is really necessary.