There can be a lot of physics going on with leaking refills.
Damaged Tip or Ball - The ball or tip (the part that holds the ball) can either wear or can get scratched from use or from dropping the pen onto the tip. Any kind of damage to these parts can lead to leaks. That's why it is important to keep the tip covered or retracted when it is not in use.
Defective Cartridge - Unfortunatly manufacturing defects can and do occur. As you already indicated, less expensive brands likely have substandard quality control and don't have a name-brand reputation to uphold.
Position - Generally it is recommended to keep pens in a horizontal or vertical position with the tip-up. Gravity works against you when keeping a pen in a tip-down position.
Temperature - Storage can be important in avoiding leaks. Cartridges are best kept in a cool, dry place to avoid excessive heat. Heat reduces the viscosity of inks so it works against you.
Air Pressure - The changes in air pressure such as those that happen when traveling by airplane can cause leaking in cartridges.
Capillary Action - Cartridges primarily work through capillary action. The ink is drawn out of the tip and onto the surface that the pen touches. Because capillary action does not rely on gravity, this can happen even if the pen is poiinting upwards, and especially if the tip is in contact with some kind of material.
My guess for your particular gooey mess, it is as you alluded to, a cheap cartridge which was perhaps further aggravated because of a combination of the above -- every little bit hurts!
Dave
