Mac Mini automatically safe boots?

I have a Mac Mini (2010 Model, last) 2.4 GHz Intel Core 2 Duo and upgraded to 8 GB of RAM (which I installed in myself). It's been running great so far, except for one problem. Sometimes when I shut it down (the proper, obvious way) at night, the next day, I turn it on and it stays forever in the white screen until a bar (similar to the first boot up) shows up and loads (slowly), and finally, it shows me my log in box but it says it's running in safe mode. I then proceed to restart it (properly) and like magic, it boots up normally! I would like to know why this happens since I know it's not due to a power failure or anything like that while I sleep. Also, a possible fix to this, so it only boots in safe mode when I tell it to. Thanks!