In Wall-E, Wall-E is made of a rusty metal material and Mater from Cars is also made of rusty metal. If you look closely at Wall-E's arms, (not the one on this screen), his arms are the same shape as Mater's rear-view mirrors. Mater's hubcap is the same shape and around the same size as the largest gear in Wall-E's wheel. Mater's headlights resemble Wall-E's eyes very well as they are both a dark gray (when the headlights are off) and round with a smaller circle in the middle. Wall-E is a cube and Mater is a little boxy. Once Wall-E needed to be recycled, his parts could have become Mater.
Theory by Rory Meyer