1. make sure the card is absolutely flat. Can be tested quite easily: turn is on its side, shoot again, if the unevenness has also turned sideways, then the card is most likely bend.
2. might be reflectivity of a black stand, exacerbated by the lights positioned slightly high, and possibly a processing that exaggerates (auto) contrast. Same test as above: if the unevenness remains at the bottom of the picture, then this is the most likely cause...