Why are these drag item objects moving on their own?

To provide a counterpoint to Scott's observations,he really is correct concerning accessibility. On the other hand, drag and drop can be very valuable if used formatively, rather than summatively. Positioning and sequencing come to mind as two powerful uses that can be difficult to  replicate by text based questions.

In your particular case, you have created motion paths to move the dropped objects to their original positions, but to the system, they are still in their dropped position. That can only be changed by the learner dragging them. So when the cursor picks up an object, the system moves it from where the system thinks it is (the last place it was dropped), rather than where the motion path moved it.

To create something that functions the way you want it to, you have to build the interaction completely from scratch. The built-in Drag and Drop just isn't that flexible.

There is a sample of a system that can do all you ask for (and some other stuff besides,) at this post: https://community.articulate.com/discussions/articulate-storyline/drag-and-drop-solution-sample-with-multiple-correct-targets-with-drag-off-and-return-plus-a-lot-more 

There is a simpler (but not as powerful and flexible,) sample attached here. Bear in mind these both are designed to show technical possibilities, but are not intended to illustrate sound pedagogical methods.