Last summer LeBron James returned home to the Cleveland Cavs, changing the Cavaliers, the Heat, the East, and even the NBA. However, his "Return" has had a much rockier start than many had anticipated. What many had projected to be the big year for Cleveland has transformed into a big flop. This has effected the east in a hugely negative way.
With LeBron in an awkward position dragging along with Kevin Love and Kyrie Irving, the Cavaliers are no longer in the picture for NBA champions. The LeBron-less Heat don't have a chance for a fifth-straight title appearance. The leaders of the east, namely the Hawks and Raptors, have little to no hope of being victorious against the like of the Rockets, Trail Blazers, or Thunder.
Without the Heat to stride through the Finals (with the exception of the 2014 Finals) the east would have to be blessed with a miracle for a title for the next few years. Trades from the east to the west also increase the west's chances for domination. This includes Josh Smith to the Rockets from the Pistons (although not too much of a factor), and Iman Shumpert and J.R. Smith to the Caveliers from the Knicks.
Tanking has become more popular among NBA teams, but even more obvious in the east, which leads to many teams losing, and losing a lot. Tanking gives the non-tanking teams an easy path to the Playoffs, which gives the non-tanking teams an astonishing surprise when they are faced with a significantly more skilled and more prepared west team.
All of these factors are commencing the fall of the east.