Drupal 8 and certain contributed modules may increase the size of headers sent out with each response. On Acquia Cloud, the soft limit is 60 individual headers and/or a combined total of 23kb of header data. Requests which exceed the soft limit may serve 503 status codes from Varnish and 200 from the Apache backend.
We are working towards a Varnish configuration change that will remove this soft limit. In the meantime you may need to adjust your site to emit less header data.
Content Translation module Link response headers
The Content Translation module exposes link relations both at the HTTP level (Link response header) and the HTML level (<link> tag in HTML <head>). This can easily exceed the number of allowable headers. The particular Link headers are generated because of this code within content_translation_page_attachments():
$page['#attached']['html_head_link'] = [ [ 'rel' => 'alternate', 'hreflang' => $language->getId(), 'href' => $url, ], TRUE, ];
To alter the module behavior so that it no longer adds a Link response header, create a custom module and implement hook_page_attachments_alter() to set the boolean to FALSE for all html_head_link elements.