What is causing this to happen?

 public class Selection_Control : MonoBehaviour {
     
     // Variable to hold a selected gameobject    
     public GameObject activeObject;
 
     
     
     // Update is called once per frame
     void Update () {
         
         // Delcare a ray from the camera to the mouse input
         Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
         RaycastHit hit;
         
         // Get the mouse position
         Vector3 mousePos = Input.mousePosition;
         
         // Right click to select a game object.
         if (Input.GetMouseButtonUp ( 1 ) )
         {
             // Ignores gameobjects that are tagged as "Static"                        
             if(Physics.Raycast(ray, out hit, 100) && !hit.collider.gameObject.CompareTag("Static"))
             {
                 // Draw the ray
                 Debug.DrawLine(ray.origin, hit.point, Color.green);
                 
                 // Assign the hit.collide.gameobject as out acitve object
                 activeObject = hit.collider.gameObject;
                             
                 Debug.Log( "Selected " + activeObject );
             }
             else
             {
                 Debug.Log("This is a static object");
             }
                 
         }
         
         //Check to see if the active object still under the mouse
         if(Physics.Raycast(ray, out hit, 100) && hit.collider.gameObject == activeObject)
             {
             
                 // Draw the ray
                 Debug.DrawLine(ray.origin, hit.point, Color.green);
                 
                 // Check to see if the left mouse button is held down.
                 if ( Input.GetMouseButton ( 0 ) )
                 {
                     // Update the active objects position with that of the mouse's position
                     activeObject.transform.position = Camera.main.ScreenToWorldPoint( new Vector3( mousePos.x, mousePos.y, 5.6f ) );
                     
                     
                 }
                             
                 
             }
         
         
     
     
     
         
     }
     
     
     
         
         
     
     
 }

I was messing around and made this script to allow selection and movement of objects via the mouse. It works fine with objects that don't have a rigid-body component but those that do have one behave strangely.

When I move the object around with a rigid-body after releasing the object it will sometimes fall through the terrain for no reason. I have checked and both the terrain and the object have colliders on them.

If it touches another object it will shoot off in a random direction and rotation, sometimes it will do this even when it doesn't hit anything else.

What am I doing wrong here ?

Thanks