It happens from time to time. You can also press ALT-CAP-DELETE this is called a soft reset and stops all applications on the BlackBerry. What you did a battery pull is called a hard reset and simply does just that...resets the device.
Also, make sure you don't have a lot of apps running in the background. Close any apps you use with Menu> Close. There will always be 5 running in the background and these include: Phone, BBM, Messages, Browser, and Home Screen. You cannot close these 5.