The reason for the extended Season 3 was because Blizzard had wanted PvP progression to match PvE progression, so were waiting for more guilds to gear up with Sunwell-level items. Considering how difficult Sunwell Plateau was, this took some time, delaying Season 4 considerably. PvE content in Wrath, on the other hand, was so accessible that by the time Season 5 started a month after release of the expansion, players were decked out in the best PvE gear -- which changed the balance of Arena play for the first few weeks.
Blizzard is looking to address that by starting Season 6 simultaneously with the "first tier of raid content" or essentially make PvP gear more accessible to prepare for the next season of Arena play, according to Ghostcrawler. Although Blizzard hasn't given a starting date for either Season 6 or Patch 3.1, the new Arena seasons are looking to match the shortened Season 4 in length and might indicate a trend in PvP season progression. That is an exciting change of pace and a comforting message to PvP enthusiasts that Blizzard is committed to keeping PvE and PvP progression at a similar pace. And if the development of Ulduar is any indication, that seems to be a fast one.