The young ones (and most newly aquired ones) go into the self preservation/defense mode after ingesting a meal. They get defensive because with a heavy load on their gut they are less mobile and very vulnerable. They will be quite jumpy initially and flutter that tail to scare you off. If that doesn't work and the threat continues they will make another attempt at running... if that fails they will generally regurgitate that meal quicker than anything you have ever seen shooting out of a snake before and run like the wind.
Be quiet, gently touch the little one until it settles down, letting it know that you are not going to harm it and eventually it should let you pick it up. It may still be quite touchy for a while so immediately and gently put it back in it's enclosure. After a few episodes of this it should get much calmer with you after feedings. The older they get and the more they get to know you, the more relaxed they become. By the time they are sub-adults they know you and the routine, you should be able to pick them up immediately after they eat with no defense response. They will even chomp down a meal while you hold them and be perfectly happy.