The origin of the resistivity anomaly at 120 K in the layered pnictide oxide Na2Ti2Sb2O is discussed on the basis of tight-binding band structure calculations. The Fermi surface of this material is shown to result from the superposition of a two-dimensional and four pseudo-one-dimensional components. It is proposed that the resistivity anomaly is associated with a Fermi surface instability of these pseudo-one-dimensional portions leading to a periodic lattice distortion.