口 (the mouth radical) marks 唆 as a verbal action — instigation works through speech, whispered urging or open egging-on. It groups with other mouth-action chars like 喊 (shout), 喷 (spurt), 吵 (quarrel) in the speech-and-noise family.
夋 supplies the sound, drifted from qūn to suō. The same phonetic stem feeds 俊 (jùn, handsome), 峻 (jùn, steep) and 骏 (jùn, fine horse) — the j-/q- and s- branches of this family run side by side.
