Food rots before expiry date

My husband went to to Woolworths and purchased two boxes of Malva Pudding as we were having guests. We opened the one packed, which was fine and left the other packet in the fridge. However, this morning when I wanted to give some to my ailing mom, it had mold on, even though the package was still sealed and the expiry date on the outside of the package is the 22 February! How does this happen?