Assuming no one has monkeyed with the electrical system.
The power for the starter has its own path seperate from other stuff. So if it happened while attempting to turn the engine over, the problem was too much resistance for the amount of current. The two most likely causes are not exclusive, bad cable terminal and failing starter.
For more about how current flows when the key is turned to start, see this post.
Ground path for the starter is the transmission housing and engine block to the ground cable.