I have made an animation to explain the cause of the slump, or at least, what I think happened!
I visited the site of the landslide to take some photographs, which can be seen at Flickr.
More information about the Jurassic coast here. Southampton University have a number of pages devoted to the geology of the region and this site has some useful visualizations about the mass movement processess operating locally. Fossil hunters might be interested in a description of the fossil-bearing strata and field guide found here.