Tree-wood at the bottom anchors the imagery: young, green wood that yields and springs back is the source of the abstract sense soft / supple / yielding. The radical also files 柔 with other timber and pliancy chars.
The spear on top carries the phonetic role, with máo drifting to róu. It also feeds the meaning: a spear shaft was made of pliable wood that could bend without snapping, so 矛+木 evokes a supple wooden weapon.
