Coincidence that the hashrate dropped when marshals raided BFL, so perhaps their customer rig "testing" mining farm went offline?
This is very well possible. The FTC however alleges that BFL "only" tested customer miners for two days prior to shipping so based on this the BFL farm should not have been that large. (I do suspect that the actual testing period could be accurately measured in months).
The miners do not actually lower the difficulty, they can only take mining capacity offline which will cause it to take longer for blocks to be found which will generally cause the difficulty to increase (assuming the lower hashrate is low enough to it take more then 2 weeks to find 2016 blocks)
