Just like any car attempting to pass another slower car, it is their responsibility to make sure they can do it safely. Many bikers seemed oblivious of the cars around them. There are times when I think they ride far out into the lane to prove a point/soothe a chip on their shoulder, but on this trip it seemed to be simple carelessness.
*Stepping on soapbox*.I think bike riders should be required to have and use mirrors on these large bike events. I have been wearing one for the past year or two, and it makes a tremendous difference in my ability to ride safely, particularly on these larger rides. I am aware of both bikes and cars approaching and can make any adjustments necessary to allow either to get by.
When a faster biker is approaching a slower one, they need to check to make sure if it is safe to pass. If it isn't, tuck in behind and wait until it is.
*Stepping off soapbox*.
For those looking for some mirrors with a little flair, I saw these on the STP last month. It is one of those stupid simple ideas that almost anyone could have thought of (over a beer). But this San Diego company actually followed through and I hope they make a mint. The wire is made from a recycled bicycle spoke, and you can even send in your favorite bottle cap if they don't already stock it.