火 depicts flames rising and supplies the burning sense - 灼 means to scorch or sear. It puts the char in the fire family alongside 烧, 烫, 炒 and 烤, which all involve heat or cooking.
勺 sháo gives the sound, drifted to zhuó in 灼. The same phonetic shows up in 约 (yuē), 的 (de/dí), and 酌 (zhuó), so the zhuó reading travels cleanly with the 勺 family rather than the sháo branch.
