I need some help with boot camp on mac...

Okay, so I attempted to do boot camp, and it worked moderately well with windows on it. However after a little I decided to remove it. I did this by deleting the partition and having only one. Then upon start up after updating, I got to a screen that looked like a windows one that said to boot cd. After restarting again and holding the option key…I selected the mac partition ( the only one there) and it worked. Any help/reasons why this happened?