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The quasi-one-dimensional (QOD), many-hard-disks system is a model of the motion of N circular particles within a long, narrow two-dimensional channel. Interaction consists of elastic hard collisions between neighbouring particles. The curvature of the interaction surfaces and the symmetries of the dynamics give the system a number of properties, which we investigate by the numerical calculation of its Lyapunov spectrum and Lyapunov vectors. We focus on the Lyapunov modes, which are the Lyapunov vectors corresponding to the near-zero part of the Lyapunov spectrum. We attempt to explain the form of the Lyapunov modes purely as a result of the structure of tangent-space dynamics, using a phenomenological field theory approach. The basic structure of the dynamics is an exchange of a fraction of the difference between the corresponding tangent-vector elements of a pair of colliding particles. This structure leads to the different Lyapunov mode forms, which we treat as fields, given the appropriate restrictions. The time-constant sinusoidal transverse modes are due to a minimisation problem. The same applies to the longitudinal and momentum-proportional parts of the LP modes; the wavelike motion is produced by the coupling between the two fields. We verify these ideas by introducing a toy model which reproduces the basic structure of the tangent-space dynamics by replacing certain expressions involving phase variables with random numbers.