Could be a latency issue where the client thinks you're still and lets you bind, but the server sees you as still moving and fails the attempt.
It happens occasionally if you try to cast a spell immediately after stopping. I'm sure any number of Druids and Wizards can tell you about it, as it happens quite often if you don't pause for a split second after stopping when trying to cast while kiting.
Try to replicate the problem while standing completely still, then try hitting your bind button right as you let go of your movement key.
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