In Generation VI, during Horde Encounters, Trace will copy the ability of the Pokémon on the far left, moving down the row to the right if the initial Pokémon in that position has been knocked out. [Source.]
Meaning, instead of being random in double battles, the abilities would copy in this order:
2 3 4 | 1 2 3 4 5
1 x 5 | x
The "x" is your Pokemon.
To answer your question: Trace only shows the Pokemon on the left's ability at first.
Source: Experience and above link.