Its no error, after every season finishes all achievements for that season are reset (MMR,Rating,Teams,Currency Earned in a Season Count), Currency Honor/Conquest is also reset (Conquest is technically converted to Honor and Honor over cap to Gold) alongside it if new gear is released (This will never happen on WotLK/Cata becase they are on their last/high end gear, and will happen to MoP only once in the future).
You may or may not like it, but it is as it is.
It happens in any game with any kind of ladder.