If the one that was attacked was just an ordinary mine, it might not be a big deal. However, the mine that was attacked and looted was a Tera Mine that had hundreds of tons of Tera ore stored in the warehouse at the time of the attack.
During the attack this time, the Tower of Dusk's direct losses amounted to billions of gold coins. Moreover, with the situation where there was no market for the Tera Mine, one might not be able to buy hundreds of tons of Tera ore even if they could afford to offer billions of gold coins.
Not long ago, the Tower of Dusk had just established their might and asserted dominance using some brutal and bloody means. If someone dared to attack the Tower of Dusk under such circumstances, that would be an insult to the Tower of Dusk. Hence, after the news got out, there was a huge uproar in the Tower of Dusk, and the other forces of the Breezy Plains were frightened too.