In his article at Sharps Pixley, Gold Crushed by 400 Tonnes of $20 billion of Selling on COMEX, former gold trader at NM Rothschilds & Sons and Credit Suisse, Ross Norman, describes how the ambush was carried out:

The gold futures markets opened in New York on Friday 12th April to a monumental 3.4 million ounces (100 tonnes) of gold selling of the June futures contract (see below) in what proved to be only an opening shot. The selling took gold to the technically very important level of $1540 which was not only the low of 2012, it was also seen by many as the level which confirmed the ongoing bull run which dates back to 2000. In many traders minds it stood as a formidable support level... the line in the sand.

Two hours later the initial selling, rumoured to have been routed through Merrill Lynch's floor team, by a rather more significant blast when the floor was hit by a further 10 million ounces of selling (300 tonnes) over the following 30 minutes of trading.
This was clearly not a case of disappointed longs leaving the market - it had the hallmarks of a concerted 'short sale', which by driving prices sharply lower in a display of 'shock & awe' - would seek to gain further momentum by prompting others to also sell as their positions as they hit their maximum acceptable losses or so-called 'stopped-out' in market parlance - probably hidden the unimpeachable (?) $1540 level.

The selling was timed for optimal impact with New York at its most liquid, while key overseas gold markets including London were open and able feel the impact. The estimated 400 tonne of gold futures selling in total equates to 15% of annual gold mine production - too much for the market to readily absorb, especially with sentiment weak following gold's non performance in the wake of Japanese QE, a nuclear threat from North Korea and weakening US economic data. The assault to the short side was essentially saying "you are long... and wrong".
Futures trading is performed on a margined basis - that is to say you have to stump up about 5% of the actual cost of the gold itself making futures trades a highly geared 'opportunity' of about 20:1 - easy profit and also loss! Futures trading is not a product for widows and orphans. The CME's 10% reduction in the required gold margins in November 2012 from $9133/contract to just $7425/contract made the market more accessible to those wishing both to go long or as it transpired, to go short.
Soon after we saw the first serious assault to the downside in Dec 2012, followed by further bouts in January 2013 - modest in size compared to the recent shorting but effective - it laid the ground for what was to follow. One fund in particular, based in Stamford Connecticut, was identified as the previous shorter of gold and has a history of being caught on the wrong side of the law on a few occasions. As badies go - they fit the bill nicely.