Microsoft Security Code - what's the trigger?

Question

I'm just wondering what the trigger is for MS to generate a Security Code email?  Why was I able to log in without issue earlier this morning, but was asked for a security code when I logged in later the same day?  Here's the email email I received after I followed the prompts:

Answer 1

Hi Krista,

If you receive an email allegedly from Microsoft please check this way to see if it is genuine.

(Your above copy of what you received does not seem to come from Microsoft.)

Microsoft usually has a green shield on the sender's name.

and you might see a lock in the URL / address line

These indicate that the email is coming from Microsoft /  legit sender.

*** Please check your email to see if the green shield is there. ***