It happens with any ability that uses that particular raise-your-arm-straight-into-the-air animation, while using sword-and-board. For nightblades, Siphoning Attacks will do it, but it works for other abilities, like Igneous Weapons on a DK. And only if you're on a sword-and-board bar.
This bug (or quirk or whatever) has been around for a good long while, though. I think it was introduced all the way back in Orsinium last year.