Why folders keep shifting their positions?

Question

I bought a new MacBook Pro 15-inch model with highest specifications in this June. I also installed newest version of Mac OS X, Lion (updated).

Yesterday, I turned on my computer and noticed that some of the folders on my desktop were rearranged. Today, the same thing happened again. I would like to ask you what the possible problem could be?

I did not touch any of the ".DS_Store" files.