Character Creeping Backwards

Hey, I am fairly new with Unity, but I have made character controllers before and never had this issue. Anywhere between 5 seconds and 3 minutes after hitting play, my character will begin slowly creeping backwards. It happened with the Standard Asset First Person Character Controller so I followed a tutorial to make another one and the bug followed me. The bug doesn't seem to be activated by any particular action, as I can just wait a minute or two without moving and the character will inevitably start slow walking backwards. I cannot for the life of me figure out why this is happening and would love some help.

Here is my code:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class PlayerMove : MonoBehaviour
 {
     [SerializeField] private string horizontalInputName;
     [SerializeField] private string verticalInputName;
 
     [SerializeField] private float walkSpeed, runSpeed;
     [SerializeField] private float runBuildUpSpeed;
     [SerializeField] private KeyCode runKey;
 
     private float movementSpeed;
 
     [SerializeField] private float slopeForce;
     [SerializeField] private float slopeForceRayLength;
 
     private CharacterController charController;
 
     [SerializeField] private AnimationCurve jumpFallOff;
     [SerializeField] private float jumpMultiplier;
     [SerializeField] private KeyCode jumpKey;
 
 
     private bool isJumping;
 
     private void Awake()
     {
         charController = GetComponent<CharacterController>();
     }
 
     private void Update()
     {
         PlayerMovement();
     }
 
     private void PlayerMovement()
     {
         float horizInput = Input.GetAxis(horizontalInputName);
         float vertInput = Input.GetAxis(verticalInputName);
 
         Vector3 forwardMovement = transform.forward * vertInput;
         Vector3 rightMovement = transform.right * horizInput;
 
 
         charController.SimpleMove(Vector3.ClampMagnitude(forwardMovement + rightMovement, 1.0f) * movementSpeed);
 
         if ((vertInput != 0 || horizInput != 0) && OnSlope())
             charController.Move(Vector3.down * charController.height / 2 * slopeForce * Time.deltaTime);
 
 
         SetMovementSpeed();
         JumpInput();
     }
 
     private void SetMovementSpeed()
     {
         if (Input.GetKey(runKey))
             movementSpeed = Mathf.Lerp(movementSpeed, runSpeed, Time.deltaTime * runBuildUpSpeed);
         else
             movementSpeed = Mathf.Lerp(movementSpeed, walkSpeed, Time.deltaTime * runBuildUpSpeed);
     }
 
 
     private bool OnSlope()
     {
         if (isJumping)
             return false;
 
         RaycastHit hit;
 
         if (Physics.Raycast(transform.position, Vector3.down, out hit, charController.height / 2 * slopeForceRayLength))
         {
             if (hit.normal != Vector3.up)
             {
                 print("OnSlope");
                 return true;
             }
         }
         return false;
     }
 
     private void JumpInput()
     {
         if (Input.GetKeyDown(jumpKey) && !isJumping)
         {
             isJumping = true;
             StartCoroutine(JumpEvent());
         }
     }
 
 
     private IEnumerator JumpEvent()
     {
         charController.slopeLimit = 60.0f;
         float timeInAir = 0.0f;
         do
         {
             float jumpForce = jumpFallOff.Evaluate(timeInAir);
             charController.Move(Vector3.up * jumpForce * jumpMultiplier * Time.deltaTime);
             timeInAir += Time.deltaTime;
             yield return null;
         } while (!charController.isGrounded && charController.collisionFlags != CollisionFlags.Above);
 
         charController.slopeLimit = 90.0f;
         isJumping = false;
     }
 
 }