Excellent question Al. Definitely a topic that we should cover here at the ENSO Blog. It is a little bit of a chicken or the egg predicament. The trade winds can weaken due to an MJO event  or simply due to the chaotic nature of our atmosphere. The ocean state itself can also spur a weakening of the trades as well with changes in the ocean temperature gradient between the waters in the west and central/eastern Pacific. The hard part is teasing out what came first as this is all very much a coupled system. The oceans can change by responding to changes in the wind which then can cause a further changing of the winds, which can then cause a change in the ocean and so on. So to answer youe question, it is somewhat related to the piling up of warm water in the western equatorial Pacific but it could be caused by something as simple as natural variability as well.