I believe according to wk057, it seems many of those failures are due moisture via vents and the AC drain being on top of the packs. Later builds mid 2015 and newer with design changes, seem to be doing much better even in their point in history.
I suspect failure rate will be less in the future with newer designs.
There are some definite trends with early battery packs failing in earlier Model S vehicles... from day 1 builds up to about Q4'2014 and some even Q1'2015... and any cars that have had Tesla-refurbished packs installed. The issues are all pretty much the same thing in one form or another...
teslamotorsclub.com