any idea why this happens?

extends KinematicBody2D
const ACCELERATION = 500
const MAX_SPEED = 100
const FRICTION = 500
var velocity = Vector2.ZERO
onready var animationPlayer = $AnimationPlayer
onready var animationTree = $AnimationTree
onready var animationState = animationTree.get("parameters/playback")
func _physics_process(delta):
var input_vector = Vector2.ZERO
input_vector.x = Input.get_action_strength("ui_right") - Input.get_action_strength("ui_left")
input_vector.y = Input.get_action_strength("ui_down") - Input.get_action_strength("ui_up")
input_vector = input_vector.normalized()
if input_vector != Vector2.ZERO:
animationTree.set("parameters/idle/blend_position", input_vector)
animationTree.set("parameters/walk/blend_position", input_vector)
animationState.travel("walk")
velocity = velocity.move_toward(input_vector * MAX_SPEED, ACCELERATION * delta)
else:
animationState.travel("idle")
velocity = velocity.move_toward(Vector2.ZERO, FRICTION * delta)
velocity = move_and_slide(velocity)