Seattle had loaned Montano to Montreal, which created the roster spot the club used to sign Sammy Ochoa. When the loan ended, the Sounders decided to keep the roster spot rather than adding Montano. Therefore, in effect, he goes to Montreal for a roster spot.
Here's the news from the Impact.
Here is the portion that deals with his time in Seattle:
Montaño, 20, was loaned to the Impact by the Seattle Sounders FC, from Major League Soccer, on August 15, 2011. In six games with Montreal, including five as a starter (413 minutes of play), Montaño registered one goal and two assists. Before joining the Impact, he played two games with the Sounders' first team in 2011 and eight games, including five starts, with the reserve team, tallying one goal and two assists in 465 minutes. In 2010, he played 12 games with the Sounders, in all competitions, including six as starter, tallying two assists in 535 minutes of play. During the previous season, he scored 13 goals in 19 games with Club Atlético Quilmes, in Argentina. In 2008, he scored 18 goals in 24 matches with the reserve team of Colombian club Envigado FC.