A classic problem encountered is when a product goes from traditional materials such as metal, wood or ceramic to plastic. One for one replacement of the geometry often brings with it inherent problems. In this case a cap formerly made of metal retained all the details of the original metal part including sharp notches, and wall thickness. Additionally the plastic chosen was very notch sensitive.
Notches in parts under stress act as a multiplier of that stress and often lead to part failure. Non-destructive CT imaging can identify hidden notches as seen here.