One one rocket blew up this year, and one last year (two rockets in two years). The reasons for both are known with over 99% certainty. (CRS-7 was failure of a helium tank strut, AMOS-6 was formation of crystals of solid oxygen within the COPV overwrap).
Both NASA and SpaceX investigated the CRS_7 accident and disagreed about probably cause. SpaceX went with the NASA report and implemented ALL of NASAs recommendations.
Dragon 2 shares a common design with Red Dragon and there is no way Elon will allow that slip because Red Dragon can only launch on certain dates and missing 2018 would mean a delay to Red Dragon of 27 months.