soldier, private, die
卆 centers on a low-ranking soldier or private, and by extension the abrupt end of a soldier's life: dying. The meanings are connected through the idea of a common soldier whose fate is often to die in battle.
卆 is a variant or simplified form of 卒. The historical development of 卒 is complex, but 卆 likely arose as a cursive or abbreviated shape. The exact origin of the 九-over-十 structure is uncertain.
Picture a soldier made of two parts: 九 (nine) above 十 (ten). A private is just one of ninety-nine others, and his life can end abruptly—like a countdown from nine to ten.
For ソツ, imagine a soldier who is so clumsy he trips over his own feet: "so tsu-pid!" The silly phrase "so tsu-pid" cues ソツ while the soldier stumbles.