This happens if the server sees something out of the ordinary from the program that could be something like a hacking attempt or blank session response. There are some tips and tricks on how to make sure your browsers are set up correctly to ensure this happens less often. 

https://accountants-community.intuit.com/articles/1615898-mfa-resolving-the-i-m-not-a-robot-message

I even get it myself every few weeks.