Maybe

The shock rendered Evie immobile as her eyes were fixed to the dragon that was emerging from the thick dark clouds. Her mind and her heart were trembling in fright because of the fact that the dragon was already called. How can this happen?

Evie knew that calling the dragon was always the last resort. The dragon guardian would never call the dragon immediately because it was always their secret weapon. Apart from that, the main reason was because the guardians cannot keep the dragon for a long a time nor could they control the dragon however they wanted to. Dragon guardians do not have full control over the dragons they summon as opposed to general views. Thus, it was always a rule that the guardians would only call on the dragon when it was time for the final battle or when they were sure they were on the losing end.