To elaborate on what Chubbz said, hunters have this unique problem of needing to stop TWO things. The first /stopcasting stops our auto-attack (which is what melee use /stopcasting macros for), and the second /stopcasting stops our current cast (which is what ranged use their stopcasting macros for).
Any time you want to stopcasting a CoS/SS, you need the double /stopcasting.