This happens with most weapons, but is very pronounced in longer weapons (2h and polearm animations briefly tested). Any type of small incline or irregularity in the map can trigger the 'collision with ground' bounce, forcing you into a half-second animation, effectively stunning you. After talking with Shik, I know this isn't intended behavior.
Ground collision is a must, but perhaps removing the 'ground bounce' trigger on overheads or shortening the animation to stop a little higher than perpendicular would alleviate the issue. Either way it's low priority.
