Being a user with good reputation doesn't mean you will never see a CAPTCHA.
It does mean that you won't see it as much as a new/anonymous user.
There are a lot of different factors that go into when it comes up (including posting many posts in a short time period).
As Oded mentioned, there are a number of factors that may cause a CAPTCHA to appear. One of these is time. Perhaps the time was very short.
I've typed up answers while riding the bus, and switching to a different network before posting made CAPTCHAs appear as well. Not sure whether there might be linkage with the locality (network) when it comes to saving/...
The appearance of the CAPTCHA should be based on the The Complete Rate-Limiting Guide. I can see that the following scenarios might trigger it:
Seeing a question; writing up code that solves the problem; copy-and-paste the code only and posting as the answer and then working adding some mortar to your brick(s) to provide something more consumable.
You want ...
In general I block third party scripts, so I never see the actual captcha, but only the SX internal message that I'll have to solve a captcha. This happens to me every time I try to add an answer that contains only code I just copied pre-indented from my VIM:
I then click outside of this banner and enter a few words to the beginning of my answer inside of ...