There's some decent logic to it from what I can see. On the main page the right hand content boxes have been broken down, some have moved to the left and the navigation has moved to the top. This leaves more horizontal space for the main content, promos and so on.
Plus, the categories navigation section was duplicated on the top navigation bar on the old site, which really wasn't necessary. They've managed to clear it up and keep it to the top only so as to cut down on the need for as many side content boxes as they had.
On product info pages they've completely got rid of the left and right content boxes.
This means that they have a lot more horizontal space to play with, whilst not having to actually increase the amount of horizontal space the layout takes up.
The new product info pages are actually better in terms of function compared to the old ones. All the information is presented in a much neater format and overall it looks a lot cleaner.
In my opinion, it would be much better if
https://www.overclockers.co.uk/pc-components
was the landing page (root page, whatever). The "PC Components" tree on the left side is a much better design than a JavaScript pop-out menu.
A little JS can be used to great effect, but I think mouse-over pop-out menus are more awkward to use than static links.