Concrete basement slabs below grade will almost always have moisture in them. A single layer of tile allows moisture in the concrete to escape through the tile joints. When you place new tile over existing tile, you cover the tile joints and essentially cut off the moisture escape routes. The trapped moisture beneath the tile can cause loosening of the old tile as well as the new tile. Vinyl flooring that has been conditioned at the recommended room temperature has better installation characteristics.
All materials expand or contract when atmospheric conditions change.
Vinyl flooring is stiff when cold, flexible when warm. In order for the flooring to expand or contract with the subfloor, it must be conditioned to the same atmospheric conditions. The grade level is where the flooring will be installed in relation to the ground around the building. Normally, this would be over the basement or a crawl space. An on-grade subfloor is in direct contact with the ground, or over a fill that is in direct contact with the ground.