How can i fix weird physics bug/glitch ?

i have stange bug with physics , when unfreezing parented rigidbodies .
i have made a 1 .

my code for chopping down trees and unfreezing rigidbodies :

using UnityEngine;
using System.Collections;

public class CHOPCHOP : MonoBehaviour {
	public Rigidbody body;
	public Rigidbody[] crystals;
	// Use this for initialization
	void Start() {
		foreach(Rigidbody crystal in crystals) {
			crystal.constraints = RigidbodyConstraints.FreezeAll;
		}
	}
	
	// Update is called once per frame
	void FixedUpdate() {
		if (Input.GetKey(KeyCode.T)) {
			body.AddForce(Vector3.forward * 5);
			foreach(Rigidbody crystal in crystals) {
				crystal.constraints = RigidbodyConstraints.None;
			}
		}

	}
}

delete the rigid body from the tree crystals and add it again with a little less gravity, see if that changes anything, if you don’t want the tree rolling remove the rigid body from it, but keep the box collider