Nobody could ever know how it happened, why it happened, and why nobody noticed it coming. The only thing they knew was that a huge ancient was around and it appeared right at that exact moment.
Even those people, who were the most prepared of all soldiers out there, felt unprepared when finally seeing something actually happening on that fortress.
Anyway, after so much time training until exhaustion, so they would never hesitate when the right moment arrived, their body just moved on its own, doing everything that the protocols demanded without even thinking about it.
The Guardians weren't there just lazying around all that time, after all. Although they lived several years of peacefulness and tranquility, since their first day the veterans made them go through hell. They experienced what real pain and despair meant.
Due to the harsh daily routine and rough treatment, they had unshakable souls and morals. They wouldn't back away when facing an Ancient showing up.