One plausible mechanism for this is that SSRIs cause reversible depletion of dopamine stores. There may be other contributing mechanisms as well. The authors provide evidence that stimulation of 5HT2 receptors in the basal ganglia inhibits dopamine release. This is suggested to be one of the reasons why atypical antipsychotics, most of which have 5HT2 antagonist activity, have fewer extrapyramidal effects. It also explains EPS caused occasionally by SSRIs.
Sertraline is a fairly potent inhibitor of dopamine release.
Even stronger than buproprion if I recall correctly. It is hypothesized that in the basal ganglia there is an interaction between serotonergic and dopaminergic neurons via axoaxonal synapses. Serotonin released from the serotonin axon binds with serotonin receptor on the dopamine axon and inhibits its release of dopamine. The subtype involved here is 5-HT2. All SSRIs have been associated with motor function deterioration in depressed patients with Parkinson's Disease.