What causes pens to "explode"?

What causes cheap plastic pens to sometimes “explode”, that is, either for ink to leak out or for some part of the to break open, causing a general mess in the owner’s pocket?

I always thought they got damaged by being repeatedly put into the pocket without a lid.

When I first saw this thread title, I thought it said, “What causes penis to ‘explode’?”.
Just thought you might want to know.

Hey! Me too!

Imagine my relief when I realized my error!


Two WAGs

  1. The pen hit something and broke off the tip.
  2. The pen got too hot and the ink leaked out through the tip because of that.

Again these are WAGs.

Me three.
… but…
I thought it was a small leak that developed due to some small defect in material. then capillary action just drew out a lot of ink.

There are some pens that simply don’t fit together snugly, so the cap leaks.

We’ve all run into pens where the cap part is slightly “off” and either it’s hard to remove or keeps falling off. Same with the ball part. Not a lot of quality control in a cheapie operation. If they find a defect they don’t “do a recall” and try to find all the ones affected, they just jiggle the handle on the alignment machine and hope it fixes itself.