This has been troubling me for ages (1 year actually) ever since it had been brought out to me by my colleagues. Now I chanced upon the causes and solutions in this website.
Basically, if you find that you are selecting multiple cells in a workbook even without pressing “Shift” or “Ctrl”, check the Status Bar of your Excel Workbook. If you see the letters EXT in bold, then Excel is in extend mode. Double-click or simply press F8 to turn it off.
Another problem solved by the trusty internet.