When electricity is used the electrical terminals warm up and subsequently cool down. Over time this heat cycle can loosen the screws. In this case one of the lives has arced and blown its self out of the terminal, the other two fell out as I removed the faceplate. Get an Inspection and Test (EICR), done every 10 years to stop this happening. If you are a Landlord every 5 years or change of tenancy.