There are many reasons why a target will disappear. Most likely, there was a hunk of rusty iron who's signal changed when you moved the dirt. Could be you pushed the target deeper or even flipped it out of the hole. You will do this less and less as you gain experience. It used to happen to me all the time and I bet in the past two years I've only had 3 or 4 of these types of signals. Practice, practice, practice.
Good luck.
Chris
Good luck.
Chris