What causes the Clearomizer to leak?

LEAKING clearomizer is the cause of overfilling, not filling according to instructions, the clearomizer was pulled off the battery too hard causing the connection to loosen ( this can be fixed by connecting the clearomizer to the battery, screwing one onto the other and then pushing the two parts together as if clicking the battery into place), missing rubber seal at the bottom part of the clearomizer mouth piece, overuse, exposing the clearomizer to high temperatures.