It isn't entirely clear. There is no obvious focal lesion. Theoretically, the dorsal Edinger Westphal nucleus could be damaged near the 3rd ventricle, while the ventral portion is OK (this would cause the Argyll Robertson pupil phenomenon). It could also be a more peripheral process, as the stimulus for accomodation is far stronger than that of light response, leading to light-near dissociation. This is less likely, as the resting pupils are not tonic.