When I talked to a brewer at a local micro here he told me this happens just about every year due to the system they use for hop between the growers and brewers. I don't have all the details but the simple version as he explained it was that all of the larger brewers reserve X amount of each hop in advance. As the year progresses and the growers have a larger crop or the brewers use less of certain hops than expected they re-inventory and see what is available to sell to the public and smaller brewers without the large accounts. One of his biggest sellers is Knuckle Sandwich double IPA that has a large amount of Simcoe. He couldn't brew it for quite some time while he was waiting on the re-inventory so that he could buy some.
