It seems to be a conflict with your CSS; I copied the source of your problem pages to a test page of my own, and removing this line fixed the problem:
<link rel="stylesheet" href="http://themes.bavotasan.com/wp-content/themes/themesby_cbavota/style.css" type="text/css" media="screen" />
One thing I noticed which may help you isolate the problem: normally the dimmed area outside the cart is clickable to dismiss the cart (same as clicking the Close or Continue Shopping controls in the cart), but the undimmed background on your problem pages was also not clickable, so attempting that left the cart visible. I'm thinking there may be a z-index issue there, so the cart's dimmed background layer is winding up "behind" your page?