Make sure you turn the tractor off and remove the key before attaching or removing PTO drivelines.
Also, if you see leaking hydraulic fluid from a hose, DO NOT touch the hoses with your hands (even gloved), you can get fluid injection and end up with gangrene. Always check your hoses for leaks with cardboard noting where you put your face and hands. Only way to be sure this isn't the case is by turning off the tractor and relieving the load on the implement, loader, or backhoe.