That's a good question, but I don't think we have the answers at this time.  Was it due to small errors in the initial conditions and the inherent chaos of the system?  (See Emily's great post on this topic: https://www.climate.gov/news-features/blogs/enso/butterflies-rounding-e….) Or are there errors imbedded within our forecast models that made the forecast for El Nino too aggressive?  The answers to those questions have big implications for the predictability of tropical sea surface conditions last summer and fall, but they require careful study first.