If you mean the one in the back that’s slightly off-angle from the grid, my guess for that is that the road existed before the modern city did and wasn’t removed to create the grid. Or it might be a rail line.

Nope, that’s Diagonal Avenue, one of the cities main thoroughfares; it was part of the original design of the Eixample, intended to break the monotony of the grid, together with the north-south Meridiana Avenue.