Mike, One can only assume it was the incident with Stewart Cink a few years back that prompted the change. Since there is no definition of a waste bunker, under the rules, it must've been an attempt to add credibility to the tour.
I doubt the old sea shells would splashed the way the sand did this week. Maybe old Petey knew what he was doing when he built it and filled it with the heavier material.
Anyone who missed the conclusion today should take solice in the fact that the final two holes created more drama than all of the 72 holes played last week.