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We address the enhancement of orange-light luminescence of Mn-doped zinc sulfide nanoparticles (NPs) induced by exposure to UV light. Ultrafine ZnS:Mn NPs are prepared by microwave-assisted crystal growth in ethanol, without adding any dispersant agents. When exposed to UV light, their orange emission intensity undergoes a strong increase. This effect is observed when the NPs are deposited as a thin layer on a transparent substrate or dispersed in an ethanolic suspension. Such a feature was already observed on polymer- or surfactant-coated ZnS:Mn NPs and explained as a passivation effect. In this study, by coupling X-ray photoelectron, Fourier transform infrared, and electron paramagnetic resonance spectroscopy, we establish that this photoactivated luminescence is rather the consequence of lattice-strain effects. Indeed, our data show that UV irradiation in air promotes surface oxidation, replacing the outer sulfide layer with a sulfate one. The mismatch between the resulting outer crystallographic metal sulfate lattice and the inner sulfide one induces mechanical strains on the latter, thus partially relaxing the selection rules controlling the electronic transition from the 4T1 to the 6A1 molecular states of [MnS4]6- emitting centers. These results are relevant because they shed light on the long controversial discussions on the origin of the photoactivated phosphorescence in such systems.