ears hanging down
耷 centers on ears hanging down, describing a drooping or floppy ear shape.
耷 combines 大 (big) and 耳 (ear), suggesting the idea of large ears that hang down. The exact historical development is uncertain, but the component arrangement visually conveys drooping ears.
Picture a big 大 ear 耳 that is so heavy it droops downward: 耷 means ears hanging down.
For トウ, imagine a tow truck pulling a giant droopy ear: tow -> トウ, and the ear hangs down like it's being towed.