Because of the increase in temperature
Have you ever mistakenly left a ball pen inside your pocket, only to find out that the ink has leaked out, leaving an irksome blotch of blue on your favourite shirt? Well, all of us have grieved through that, but do you know the reason that happens?
Because the pen is in contact with the body, the body temperature causes the ink inside it to get warmed. The ink gets liquefied and oozes around the tip. At the other end of the tube is a small grease plug which also liquefies when warmed.
The adhesive force that binds the liquid to the wall also lessens, which explains why the ink is pushed out.
The tip making contact with the fabric forces ink out of the pen
In addition to the warming of the ink, the phenomena of capillary action also comes into play! Capillary action is nothing but the ability of a liquid substance to flow in narrow spaces without the assistance of/in opposition to external forces like gravity.
How many of you remember the experiment where-in the liquid ink from a pen used to spread once it came in contact with a piece of cloth or a scrap of paper?