For cats, grooming is a totally natural behaviour with most of them spending around 5%-25% of their waking hours grooming.

(Wiki)

But it all becomes a bit excessive when it takes over all the other things they do like playing, resting and eating. 

This condition is called 'Psychogenic Alopecia' and in this video Dr Karen Becker on behalf of Mercola Healthy Pets explains the condition and what you can do about it.