When your ballpoint pen is in your pocket it gets warmer because of your body temperature.
This means the ink inside becomes more liquefied and oozes out of the tip and end due to gravity.
It also means that the small grease plug at the end of a pen (which is there to stop the ink coming out) warms up and takes on the properties of a liquid.
On top of that the adhesive force that binds the liquid to the wall also lessens… More reasons here.
Good to know, I think.