He had a weaker connection to the elements because of how he used them to kill Garrosh, or so he says.
That could be nothing more than guilt on his behalf though - they've never really gone into full detail of Thrall and his powers now due to the fact they put him on the shelf for two expansions.
His mind is all that betray him. I think elements give no shit about some orc traditions about duels or other things. He feel guilty about it (why? killing Garrosh was right thing).
Imo, he will speak with mother and she will said some inspiring shit about him and viola! we have our World Shaman again.