It's because of the awful netcode on the backend that resets your animation locks based on ping. You don't notice it as much in PVE or on striking dummies because you can pace your rotation for double-weaves, but in an actual match you're constantly clashing against the long animation on potions, oGCD CC spells and other buttons you're pressing. Blame the scavenged code of 1.0 for this design, I really hope the dev team does a full port to a new engine at some point in XIV's lifespan ala PSO2.