How to redirecting add to cart form in drupal commerce programatically?
Check if you have any redirect rules enabled under admin/config/workflow/rules, which is force redirecting you to cart page.

As a solution you can have your custom redirection rule and disable the existing redirection rule, an example redirection rule which redirects to node/2 path after adding product to cart.

{ "rules_add_product_to_cart_redirect" : {
    "LABEL" : "Add Product to Cart Redirect",
    "PLUGIN" : "reaction rule",
    "OWNER" : "rules",
    "TAGS" : [ "add-to-cart-redirect" ],
    "REQUIRES" : [ "rules", "commerce_cart" ],
    "ON" : { "commerce_cart_product_add" : [] },
    "DO" : [ { "redirect" : { "url" : "node\/2" } } ]
  }
}


In case you import the above remember to change the redirect path from node/2 to your thank-you page.