stumble
蹶 centers on losing one's footing: stumbling or tripping, often with a sense of sudden collapse.
蹶 combines 足 (foot) with 厥, which likely contributes the sound and may relate to stumbling or falling. The exact historical development of 厥 is uncertain.
The left side is 足 (foot), and the right side 厥 looks like a person tripping over an obstacle. Picture a foot catching on something and causing a stumble.
For ケツ, imagine a person stumbling and letting out a sudden 'kets!' sound as they trip: kets -> ケツ, matching the stumble meaning.