How do drumlin medial moraine form?

Could be at the edges, but more likely to be a terminal moraine formation. These mounds are just the irregular clumping of clayey moraine material, and their formation is contributed to by the iceblock - caused kettles between them.

Note, Terminal Moraine is likely to have lots of clayey material in it from the ground rock flour from the glacier base. Subsequent river/stream erosion into this material is likely to leave tall near-vertical rubble cliffs.