We assume that the origin of decoherence of non-classical motional states of a trapped ion is due to the coupling of the vibronic modes with classical stochastic electric fields and to the finite lifetime of the electronic levels. Working in the dressed basis of the effective two-level Hamiltonian, we show that these interactions lead to a dispersive-like decoherence dynamics. The numerical results agree with the experimental data reported by Meekhof et al.