It could to be related to power saving.
"Count of load/unload cycles into head landing zone position.
The typical lifetime rating for laptop (2.5-in) hard drives is 300,000 to 600,000 load cycles.Some laptop drives are programmed to unload the heads whenever there has not been any activity for about five seconds.Many Linux installations write to the file system a few times a minute in the background.As a result, there may be 100 or more load cycles per hour, and the load cycle rating may be exceeded in less than a year."
This link talks about a way to reduce LCC in some WD disk. Just take a look till you get a better answer, dont try what they do.
How to Stop Excessive Load Cycles on the Western Digital 2TB Caviar Green (WD20EARS) with WDIDLE3