collide, bump into
碰 centers on forceful physical contact: colliding or bumping into something.
碰 combines 石 (stone) with 並, which likely contributes the sound. The idea of stones colliding fits the meaning of bumping or colliding.
A stone 石 on the left and 並 (line up) on the right: imagine two stones lined up and then colliding with a loud crack.
For ホウ, picture a stone thrown with a loud 'hou' sound as it collides with another stone: hou -> ホウ.
forming a pung by picking up a tile discarded by another player