This came right in the middle of the nationalisation of the coal industry, which became effective 1 January 1947. As a result the mine owners, who were receiving what was admittedly low compensation, a combination of government shortage of funds and political idealism, did no capital investment in the mines in the years leading up to this, which of course meant nothing significant had been spent since 1939. The mines had been under central control for some years, but that had concentrated just on running expenditure rather than forward investment.
There was a comparable issue on the railway, where the first couple of years of nationalisation (and last of private ownership) was marked with a number of mainstream derailments, some quite serious, due to inadequate track maintenance. It's something that can happen with major structural changes to an industry, because exactly the same thing happened 50 years later with the transition to Railtrack. As Gerry Fiennes said : "When you reorganise, punctuality drops off, safety slips. Don't reorganise. Don't. Don't Don't."