Are you having this issue with all PDF's?
I think the issues is arising because your SNAPSHOT tool is enabled. To check go to EDIT and see wherther there is check before "Take a snapshot" option.
Just try after selecting HAND tool from tool bar in Adobe Reader.
I have the same problem. The SNAPSHOT is not checked, while this snipping function turns on automatically when you drag a rectangular. It is kind of annonying when I want to select the text. I have to align the cursor carefully with the text otherwise the snipping function will be invoked. I also want to know how to disable this function.