Junction boxes must be 'accessable', a term which is as open to
interpretation as the rest of regs. General opinion seems to be that
a junction box under floor-boards, especially with fitted carpet or
wood block is not accessable.

Apparently vibrations occur at mains frequency and this can cause the
screws to loosen because electricians, unlike other engineers, haven't
discovered self locking screws or screw locking compound yet :-)