I am guessing there is some kind of parallax JS library running behind it? Except they didn't use a good parallax library that work with all major browsers. So, it is a big fail on them IMO. Because there are plenty of better parallax libraries out there and they failed to choose a good one, or they are dumb enough to build their own library that fails.

http://smashinghub.com/7-jquery-parallax-and-scrolling-effect-plugins.htm