I have had that many times. The worst was when an emergency fitting needed re-lamping. I switched the light off but not the circuit. As I took down the louvre one of the edges touched the permanent line connector which had crumbled. BANG !! (luckily the earth fly lead was attached)
Its probably due to the ultra violet given off by some types of lighting tube, as I have encountered this problem before, where a connector block has become brittle in a lamp fixture.
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