You really think the FBI talked to DPR and discussed with him whether now would be a good time to sell? They sell now because they can, the FBI are no speculators and they certainly didn't ask for DPR's permission first.
Actually they did talk with Ross (not clear on whose initiative) and both agreeed to sell. The FBI still cannot sell those coins on their own, it is not yet decided whether they belong to Ross or to SilkRoad. If they belong to Ross, they would have to wait for the verdict on Ross's trial.
I stand corrected then, thanks.

Still the point stands that Ross and the FBI aren't speculators, Ross probably agreed with it because even at the price today they are worth a hell of a lot more than when he got them so why not auction them off? It's unlikely Ross will be planning to get back into the SR business and BTC if he is declared innocent, he will probably be happy with just the cash without worrying about the volatility of Bitcoin.
They agreed back on January 27, 2014 to sell based on volatility risk. "WHEREAS, the government and Ulbricht agree that, due to the volatile market for bitcoins, the Computer Hardware Bitcoins risk losing value during the pendency of the forfeiture proceedings;"
http://www.usmarshals.gov/assets/2014/dpr-bitcoins/order.pdfHaving agreed to permit government to sell back in January, the government no longer needs any further agreement from Ross to sell. Same link: "1. The United States and Ulbricht agree that the United States may, at its sole discretion, sell any portion or all of the Computer Hardware Bitcoin, on a date or dates and in a manner to be determined by the Government."