Per the Evening Standard:
Thousands of commuters face rush hour chaos after major Tube stations were evacuated amid fears they are too busy.
Emeregncies were declared at several busy stations on Wednesday evening with King’s Cross, Oxford Circus and Euston closed to prevent overcrowding on a day of mayhem across the capital’s underground network.
Travellers also reported Warren Street and Bond Street stations were shut as they told of chaotic journeys on their way home from work.
Arsenal supporters were urged to avoid Highbury & Islington, Holloway Road, Arsenal and Finsbury Park stations ahead of their League Cup clash with Southampton tonight after a heavily disrupted Victoria and Piccadilly line service throughout the afternoon.
http://www.standard.co.uk/news/transport/tube-chaos-commuters-face-rush-hour-mayhem-after-stations-shut-amid-crowding-fears-a3409121.htmlIt happens some times. Something bottlenecks the system - in this case it's the Piccadilly Line having half it's trains out due to them needing their wheels replaced urgently (HT - Transport for London for that) and it screws the whole network.
Add in the football match and the neat fact that something like 90% of people in the UK get paid on the last day of the month and want to get their Christmas shopping done, and it's a extra load on an old system.