Chinese or NorK interference with ship NAV / electronics systems - this was a warning to our navy to stay out of territory.
A 2015 declassified intelligence report, obtained by the private National Security Archive, provides details on China’s EMP weapons as well as the plans for their use.
The report details how much of China’s military is developing EMP weapons that the Chinese plan to use against targeted U.S. aircraft carriers with regard to any future conflict over Taiwan.
NGIC Assessment non-classified doc with (partial redactions)located here:
http://twt-media.washtimes.com/media/misc/2011/07/22/ngic-emp.pdf
even an emp attack does not explain why none of the Navy personal who were supposed to be the look outs let this happen...