In message , Tom
Anderson writes

Got the Vic at Finsbury Park, heading for Euston. Got as far as King's
Cross more or less alright. Then got an announcement: "this train is
being held here, as there is planned engineering work at Victoria; the
engineers will be on the line for about ten minutes". Got off and
headed over to the Northern line, to get to Euston; as i went down the
escalator, heard an announcement that the Victoria line was now suspended.

Firstly, any idea what it was about?

Signal failure at Victoria according to the TfL realtime page.
(Also faulty train at Warren Street, although I'm not sure if that was
later).

Secondly, am i right in assuming the "planned" was a mistake?

Sounds like it: engineering work, yes, but not planned!
--
Paul Terry