I was surprised by seeing the site asking for CAPTCHA verification. I am curious that what prompts MSE to ask a user to confirm that they are not Bots. What is the intention of site to verify that user is human?
I actually had the same experience in these cases. For my prompt, I went to make a draft of my question and I leave because of my bad Internet Connection. I come back with a good one and reviewed and posted, but then CAPTCHA arrives. It's very likely that some have experienced it by this reason, especially @hardmath who has posted a comment here on his experience.
To make sure that you are not some automated system that creates spam, abuse and violating certain rules, Stack Exchange in fact brings CAPTCHA as a protection.
It seems that it is a global StackExchange policy to prompt users with a CAPTCHA if they take more than 40 minutes writing a post, regardless of all prior reputation gains or good behavior across sites.