Warping or buckling of vinyl siding panels that have been repainted.
Most likely cause is that vinyl siding was painted with a darker color paint than the original color. Dark paint tends to absorb the heat of the sun, transferring it to the substrate. When vinyl siding expands dramatically, it is not able to contract to its original dimensions.
Paint vinyl siding in a shade no darker than the original. Whites, off-whites, pastels and other very light colors are good choices. Top quality acrylic latex paint is the best type of paint to use on vinyl siding, because the superior flexibility of the paint film enables it to withstand the stress of expansion and contraction cycles cause by outdoor temperature changes.
Siding that has warped or buckled should be assessed by a siding or home repaint contractor to determine the best remedy. The siding may have to be replaced.
(image courtesy of The Rohm and Haas Paint Quality Institute)