My Obstacle Rotations Are Not Correct

Hello,

I have these giant meatballs that I want to rotate as they come down my plane; think of a meatball rolling down a mountain, rolling straight at you.

I have this script I wrote that I believe should be working correctly and doing what I want, but it is not.. Here it is:

 using UnityEngine;
 using System.Collections;
 
 public class MeatballMover : MonoBehaviour {
 
     private Rigidbody rb;      //Reference to Rigidbody Component
 
     public float speed;        //Speed, updated through script
     public float acceleration; //Every second, the speed will increase by this much
 
     public float spinSpeed; // speed at how fast I want the meatball to rotate
 
     //Executes once, when object is spawned / scene loaded
     void Start() {
         //Get reference to rigidbody, and set the speed
         rb = GetComponent<Rigidbody> ();
         rb.velocity = -transform.right * speed;
 
 }
     //Executes every frame
     void Update() {
 
         // Rotate the meatball to make it look like it is rolling down
         transform.Rotate(Vector3.left, spinSpeed * Time.deltaTime);
 
         //Add acceleration to speed, make sure it's not above topSpeed)
         speed += Time.deltaTime * acceleration;
         //Set object velocity
         rb.velocity = -transform.forward * speed;
 
 
     }
 }

Instead of rotating in place, down my plane, they rotate around each other; almost like a black hole is in the middle and they go around it; or someone stuck an Axle in the middle and they are going around it. Can someone explain what is happening? Thank you