Source

I certainly didn't expect the defense system to suddenly activate like this, but then I realized something.

The other side wasn't an invader from an upper realm.

The reason why the system had restricted the defense system for me the entire time was because of the upper realm invaders. The system didn't want the defense system taking the kills from me, which was why it had restricted the use of the defense system.

But since the other side was an elder from one of the sects and not an upper realm invader, it didn't hesitate to use the defense system.

Though I was also surprised that the system would suddenly use the defense system since it wasn't as if I was in danger in the first place.

This elder was only in the Soul Formation Realm, he wasn't even someone who was in the Rebirth Realm.