Without the 'back link', it cannot redirect you to your original search. Your 'from location' is google.com/sorry, so, once you pass the test, it sends you back to google.com/sorry instead of back to your previous activity which triggered the Captcha.
If you're coming from an HTTPS page (which I think HN forces) to an HTTP page, the referer is not sent. It is only sent for HTTP->HTTP and HTTPS->HTTPS (even cross-domain, despite the info that leaks out)
This seems to happen sporadically in the morning at my company and I speculate that it's due to a large number of employees who are querying Google at the same time from the network's IP. Google sees this as bot-like behavior and shows the capthca.
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[ 2.7 ms ] story [ 31.7 ms ] threadhttp://www.w3.org/Protocols/rfc2616/rfc2616-sec15.html#sec15...
Take your laptop to your neighborhood coffee shop as a proof of concept.