Melt fracture a/k/a "sharkskin" or "applesauce" occurs in LLDPE film.
To say that much has been written on the subject is an understatement. Entire books and dissertations abut melt fracture have been written by polymer chemists a lot smarter than me. Still there is not total agreement on the causes. Personally, I have a hypothesis that it has something to do with susceptibility to degradation in the transition section of the screw.
Going back to the early days of LLDPE http://www.brentwoodplastics.com/blog/bid/131888/LLDPE-Linear-Low-Density-Polyethylene-varieties melt fracture was just expected in butene LLD but not Octene. Carbide advocated a return to shorter L/D ratios for shorter residence time. This and granular resin never really took off.
No matter the cause, melt fracture results in a very irregular scaly texture to the film. In most cases, this is considered an undesirable quality because it detracts from the cosmetics. In a few cases such as forearm protectors it is considered a plus but i don't know why.