Imagine a 'fire' (火) intensely 'burning' something 'high' or 'lofty' (尭) above it, like a tall building engulfed in flames. This evokes the meaning of burning or roasting.
Vocabulary
焼くやく
to grill, to bake, to burn (transitive)
焼けるやける
to be grilled, to be baked, to burn (intransitive)