Leaving the tip of the pen standing in a pen stand or touching the pen tip with lint in the pocket may cause ink leakage.
When the pen tip touches something, ink is drawn out from the pen in most cases. This is the same as the phenomenon (capillary phenomenon) in which the tissues touches a little bit of water, sucking more water than the part touched.
In the case of the knock type, ink leak may occur when the knock button is pushed and the pen tip touches things in the bag or clothing etc. unnoticed like the illustration above.