Fury of the Mountain continuing to channel when using Furious Slam

Doesn't happen all the time, but occasionally when I'm using fury of the mountain to build rage then quickly do a slam, sometimes when I press/hold hotkey for slam, it just continues channeling furt of the mountain instead of switching, won't be holding down the hotkey for fury of the mountains, I hold the hotkey for slam and it will still channel fury of the mountains
Happens maybe once every hour or 2 of mapping