Pens have a sixth sense for destruction. The moment you click one open near a shirt you actually like, something shifts in the universe. The ink, which has been sitting perfectly still for weeks, decides that now is its moment. Not when you're wearing the ratty old thing you mow the lawn in. Now. Always now.
Scientists will tell you it's about temperature and pressure changes causing ink to expand. Sure. That's one theory. The other theory, which honestly holds up better under scrutiny, is that pens are chaotic little goblins who have been quietly watching you and waiting for the worst possible moment to strike.