a) From the pen tip

a-4) When sudden change in atmospheric pressure or temperature（Liquid ink ballpoint pen）

Liquid ink ballpoint pen, when air in the ink tank swells due to changes in atmospheric pressure or temperature when ink is transferred from the ink tank to the tip of the pen, the ink is held in the bellows-shaped collector. However, if it exceeds a certain amount, ink may leak out from the air hole of the pen tip when removing the cap.

＜e.g.＞

Carrying on an airplane
During a flight, the air pressure inside the machine will be as low as 0.8 atm, but the pressure inside the pen is the same as the atmospheric pressure (1.0 atm).
Since the air pressure in the ink tank becomes relatively large, when you remove the cap inside the airplane, ink is spilled out from inside the pen.

Leave in a car or in the vicinity of the heater
If the temperature of the pen rises suddenly, the air in the ink tank expands and ink may leak out when removing the cap.

Impact and centrifugal force
In case some ink impacts or a strong centrifugal force is applied to the pen tip, when the ink exceeds the collector's holding amount, the ink accumulates in the pen tip and blocks the air hole.
If the temperature of the pen rises in this state, the air in the pen will expand and the ink may spill out.