Turns out it’s the social plugin (for this particular one) is causing the issues, but I am also assuming the other page that you said still does it is the iframe (which can be troublesome).
How this theme works with equal height columns is that a script calculates the height (length) of the longest column and applies this height to select containers (columns) so that each column is the same. So if your main content has more stuff in it, as in actual content, it will be added where the sidebar also gets that height.
IFrames are bit troublesome because they are kind of like adding a browser window into your browser window. The content in an iframe is located somewhere else, so my guess is that the equal height columns are not working well with an iframe as well. Not something you want to hear I know.
I took your content from that Giveaway page and put it into my test local site and disabled that social plugin, and was able to get all the content into the page, but not sure why yours is still showing it breaking out (perhaps browser cache is storing it). Try clearing your browser cache and see if that works.
As for a social plugin…I’m trying to remember what i used once before which worked great. I just have to look for it…which I will do shortly.
There is also the method of my theme’s equal height columns that is also a part of the cause too, so hopefully I can remember and find that social plugin I used before in the past. Are you looking for those big sized buttons?