Just like the At the Drive-In song Embroglio, this song is also about the suicide of Julio Venegas and the condemnation of the person who pushed him to the edge.

That person is Ben Rodriguez, a touring guitar player for At the Drive-In that replaced Jim Ward for a single tour before being kicked out.

Cedric Bixler-Zavala clarified that in an interview from 2002, in Leeds Cockpit:

we have a song called Concertina, and that song is about […] Julio Vanegas, he took his life for a certain reason, and someone pushed him over the edge to do it, and I know that person, and this song, to me, is a way of calling out that person and its basically accusing them of killing our friend

MR – Does that person actually know that the songs about them?
CB – No. That person actually used to play in At The Drive-In and we kicked him out mid-tour.

MR – Oh, was that Ben?
CB – Yeah, it was Ben.

Sources: [1] [2]