its called animation lock. Even though its OFF GCD you still have to wait for your animation to end from what ever skill you just used before you can use the next ability. Even if its Off GCD. Each animation has about 1 second duration. So say you use Rage of halone and you have 2.5 second GCD 1 second after you cast RoH you are able then to use 1 off GCD skill. .5 second later you can use a GCD skill and soforth and so on.