It's the widest line of the conventional planes
Last Sunday, thanks to Gerard Piqué's Periscope video, we could see Barça players on the plane. Leo Messi was sitting with Pepe Costa (a club's employee in charge of helping the players and also a personal friend of Messi) and Javier Mascherano. In fact, we could also see that Messi was sitting in the emergency line.
According to Xavi Campos (journalist of Catalunya Radio), this is where Messi always travels: one of the seats of the emergency line, since they are the widest ones. This is also good for Pepe Costa, who usually sits with him, since he is really tall.
Historically the seats of the plane where distributed by draw. The day Messi had the chance to sit in the emergency line, he decided he would not move from there: the day he had to sit in another place, he anyway sat in the emergency line.