The move was orchestrated to prevent at least one large UK bank from going under. They were shorting gold massively - along with JPM as the largest shorter, but JPM had the backing of the FRBNY - the UK banks did not have a similar back-stop arrangement with BOE.
The UK gold sale was pre-announced, on a massive scale and done in one go - all three are designed to ensure that the spot price of gold fell immediately and stayed down - the purpose was to drive out the speculators / close out margined long positions and enable the UK leveraged banks to cover their shorts.
Thus the UK banks were 'saved' (to be subsequently bought out by Japanese and German banks if the rumours on their identity are correct), there was no run on the banks (well, for another 9 years anyway) and peace was restored to the galaxy... oh, except the remaining UK banks realised that the BOE will ALWAYS back-stop their trading losses = complete loss of moral hazard = 2007/8 financial crisis = 2008 - 2015 Great(er) Depression.
The US did the same with LTCM earlier in the decade. Brown simply did the 'wash, rinse, repeat' trick here and the great ponzi scheme continues...
