Well they dont all die suddenly, its just that there's some new problem with them that appears to cause death all of a sudden if they become afflicted - check the RHS website for more info, they may have an answer to this now, I haven't looked recently.

From what I can gather Robinia Frisia is the one most affected by problems. It seems to be as a result of the roots getting too wet over the last few damp summers. the tree struggles on for a while but as it has few decent roots it dies.