Cause of ink leakage from ballpoint pen

b) In case from the back of the pen

b-2) In case shock is applied (gel ink ballpoint pen etc.)

(Followers : Gel-like end plug moving to the pen tip side with consumption of ink)

When a strong shock is applied to the pen, a hole is opened in the follower, and it may not be able to fulfill the role of an end plug.
If you lean and leave the tip of the pen above this level in this state, air enters from the pen tip and ink may flow out at the end of the pen.