I'm not saying the bikers right by any means, but I don't think they were wrong, either. Because of their size, motorcycles can be hard to see, so by driving in packs they increase their visibility. To merge through the pack can cause a safety risk, so I can see why this would irritate the biker that yelled at the OP's DH.
It is not another driver's responsibility to make room for you between themselves and the vehicle in front of them. It is your responsibility to make sure you can fit in that gap safely; if you cannot, you don't put your car there. Can it be courteous to slow down to allow another driver over? Yes, but not doing so does not make anyone rude. How would it be any different if there were three or four semis in the right hand lane blocking the OP's DH from merging over? If the gap between them was not sizably safe for the OP's DH to merge, what would he have done? Merge through them or slow down to move in behind them? With motorcycles, that safety gap needs to be a bit larger (hence why the biker was probably angry). I just don't see how the bikers were SS; to me, it is no different than any other traffic.