When they blocked Toz's sword, the death mage concentrated the power of its defenses on where the sword was, which meant that the rest was weakened instead. Since they had just attacked Hassandra with a swarm of death magic, they weren't worried about getting counterattacked while its defenses were down.
However, Scrael and Asilean were outside of the death mage's calculations, and when the two cats appeared, it was already too late for the death mage. The two cats were already right next to the death mage when the mage noticed them.
The death mage was startled but not startled enough to panic and let go of Toz's sword. Instead, they expended more mana to reinforce the entire barrier. But for some reason, their mana refused to follow their instructions when it came to where the two cats were. The barrier weakened instead of growing sturdier.
