There is some JS on the page that is adding a height to the center-leftcontent and each of the outerwappers (art-content), when the page reaches a certain height.

In my case it's adding a height of 604px. You might want to go through the script.js and find the JS in question and consider possible fixes or just remove it.

All I can tell you is that the issue is caused by the script.responsive.js file. You could disable or remove the file, but that might stop your skin from being responsive. I didn't read the whole JS file cause it's pretty big, so I don't know what else might be in there.

So you know, it is quite possible to build a responsive mojoPortal skin without all that JS. You can do it entirely with CSS.