We don't know for sure but usually it's for at least one of the following reasons which they view as a red flag:
sending more email than usual- If you normally send roughly 10 emails a day and all of a sudden send out 100, they may block you.
sending too many emails to invalid email addresses
logging in from improbable locations in a short amount of time (logging in from the US and then a short time later logging in from Africa etc)