Although drought conditions have preceded dieback events in southern states, it does not fit with what's been going on in Cape Cod. Perhaps the best evidence is that vegetation losses on Cape Cod are ongoing and continue to occur in the absence of drought. For example, the photos below show the recent disappearance of S. alterniflora between 2004 and 2008 during which there were no significant droughts.
In addition, it is the creekbanks that have lost the most vegetation over the years. However, these areas are flooded daily with seawater, which negates any possible long-term effects of drought on soil chemistry (such as decreased pH and mobilization of metals).
Finally, if severe drought were the primary cause of vegetation decline, the expectation would be for large-scale, simultaneous losses in multiple marshes during a single year. This has certainly not been the case. On Cape Cod, vegetation losses began at different times in different marshes and have been progressive over a period of many years.