Just found this on http://tube.tfl.gov.uk/content/nexus/

Forthcoming Planned Engineering Work
NORTHERN LINE EARLY FINISH TO SERVICES:
On Tuesday 24 February the last Northern line trains will depart central
London at around 2230. Valid Underground tickets will then be accepted on
all existing local bus routes to connect you with shuttle services on the
following sections of the Northern line, which will continue to operate
until usual finish times.
..East Finchley - High Barnet/Mill Hill East
..Golders Green - Edgware
..Stockwell - Morden
Services will resume as usual to all destinations on Wednesday 25 February.

I'd guess there's something going on with the points at Camden Town.