Phew, too long for a comment.
The liquid of (young?) coconuts is sterile and can even be used for transfusions. The abstract of the linked article doesn't say anything about the sterility of older coconuts. I assume that the older coconuts (esperically the peeled ones) are not sterile anymore. This not very trustworthy looking website (this article is written by someone who sells coconut-related products...) says the liquid of older coconuts tastes sour.
If I search for "brown coconut juice sour" (both in English and in German) I get results (of ...well, not very trustworthy websites, too) that say the juice ferments if stored for too long. 1 Then, the highest rated answer on Yahoo Answers makes sense:
The longer a coconut stays and ages on the palm, the stronger and sharper its flavor gets...pick it early at the right age, but store it for an awfully long time, the flavor fades OR if it's a high-sugar variety, then it turns sour fast.
If I search for "brown coconut juice sour ferment" I get (among many, many results about coconut juice-based kefir) an article of the Cincinnati Herald by Timothy Moore:
The fruit itself has a very short life span and likewise must not come into contact with oxygen for too long. Fermentation will begin and the coconut and water will taste sour and smell. It will attract bacteria and complications will arise after it is digested.
The commercialization of coconut water prevents it from fermenting and souring as quickly as the natural fruit.[...]
If you are shopping for a coconut, don’t be afraid to ask how long the fruit was transported or how old the coconut is? In some cases, ask if you can open the coconut to be on the safe side. You don’t want to leave the store with a bad coconut.
On the one hand I would to take this article with a pinch of salt since the description of the author at the end of the article sound quite sensationalistic - on the other hand this explaination sounds plausible.
1 "Bei zu langer Lagerung wird das Kokoswasser sauer oder gärt." Translation: The coconut juice turns sour or even ferments if stored fo too long. Source.