...and probably everything to do with normal climate variations or climatic variations forced by anthropomorphic climate change and its effect on sea temps, and most importantly human over fishing. Sardine populations are inherently volatile, but that is greatly aggravated by humans rampaging though the fish stocks when populations are relatively high.
See the interesting graph at the end of the linked story.
http://www.latimes.com/science/la-me-sardine-crash-20140106,3994833,1114214,full.story#axzz2qUDUmBfI
After commercial exploitation crushed sardine populations by the early 50's, it took around 40 years for them to come back, but not nearly to their huge numbers in the early 20th century. Scary thing is sardines, like anchovies and other species are the base of the western Pacific food chain. Humans have wiped out the western Pacific sardine fishery and we'll have to wait decades for them to come back, if ever. So what do we do now? Lets rape squid and anchovy stocks! Smart! 
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