to bury
窀 centers on the act of burying, specifically placing a body into a grave or tomb.
窀 combines 穴 (hole, cave) with 屯, which likely contributes a phonetic element. The character is associated with burial pits or graves.
A hole (穴) in the ground with a body placed inside (屯 suggests a cluster or gathering, here of remains) represents burial.
For チュン, imagine a burial mound where a tune is hummed: tune -> チュン, as mourners gather around the grave.