step on, tread on, stumble, slip
蹋 centers on forceful foot contact: stepping on, treading on, or stumbling and slipping as unintended foot actions.
蹋 combines 足 (foot) with a phonetic component that likely contributed the sound トウ. The exact historical development of the right side is uncertain.
The left side 足 means foot. The right side looks like a jumble of things under a roof: imagine stepping on a messy pile and stumbling.
For トウ, picture your toe stepping on something sharp: toe -> トウ, and you stumble from the pain.