The origin of the huge infrared luminosities of ultraluminous infrared galaxies (ULIGs) is still in question. Recently, Genzel and colleagues performed a mid-infrared (MIR) spectroscopic survey of a large number of ULIGs and found the major energy source in them to be massive stars formed during recent starburst activity; ~70%-80% of the sample are predominantly powered by starbursts. However, previous optical spectroscopic observations classified the majority of ULIGs either as Seyferts or as LINERs. In order to reconcile this difference, we compare types of emission-line activity for a sample of ULIGs that have been observed in both optical and MIR. We confirm the results of previous studies that the majority of ULIGs classified as LINERs on the basis of optical emission-line diagnostics turn out to be starburst-dominated galaxies on the basis of MIR diagnostics. Since MIR spectroscopy can probe the more heavily reddened, inner parts of the ULIGs, and since it is quite unlikely that the inner parts are powered by starbursts while the outer parts are powered by nonstellar ionization sources, the most probable resolution of the dilemma is that the optical emission-line nebulae with LINER properties are powered predominantly by shock heating driven by superwind activity; i.e., blast waves driven by the collective effect of a large number of supernovae in the central region of galaxy mergers.