The short answer is that this happens when the system thinks, based on how long you've been editing the text (taken together with other factors), that you might be a bot.
Why would it think you might be a bot when clearly you're an established human user?
The system is designed to try to protect against harm caused by a user account that belongs to a real, ...
That is part of reCaptcha's system, I believe. And because of that, the people to report that issue to is reCaptcha, especially if it's global across all the reCaptcha things you see.
The developer's FAQ for this is here. They also have a discussion group on reCaptcha where you would likely go to report issues with reCaptcha.
Note that I don't think they ...
We actually show Captchas on sign up under certain conditions.
That being said, I'm not sure inconveniencing every individual attempting to sign up is our best approach to combat spam right now:
Captchas wouldn't necessarily keep all the spam out (if only because you can pay humans to work around them), so we can't rely only on Captchas.
Captchas would ...