In 1913 he had just sold the idea of using diesel engines for submarines to the French navy and was on his way to sell the idea to the British navy. British newspapers blamed it on the French, but no evidence was ever found, and many think it was suicide (he had a history of depressions).

I reject the suicide theory entirely because conspiracy theories are much more fun. Lacking a suicide note there is no direct evidence for suicide. On the other hand, he was known to be friendly to the French, Brittish and Americans, and the German navy was going gung-ho for diesel uboats.