Two linked senses. Physically: 崇山峻岭 (towering, steep mountains). Figuratively: 严峻 (grim, severe), as in a severe situation or stern expression. A steep slope and a harsh demand share the feeling of being hard to overcome.
Left mountain radical, the indexing component, fixing the core image: high, steep peaks. From the physical steepness grows the figurative sense of harsh severity. Same mountain family as 峰 and 岭.
