From what I can read, they are actually excecuting two binding spells. The first one is for a minor demon. When it turns out (or so they thought) that Tom isn’t a minor demon, they excecute a binding spell for a level 4 demon. That itself could have supercharged the binding.
There is also the fact that, while they are prepared for the minor demon binding spell, they do the level 4 demon binding spell in a hurry. And as the saying goes, haste makes waste. They might actually work more on the level 4 demon binding spell than if they had done the spell properly.
Then they are also surprised and scared when Tom fight back. Surprise and fear usually cause our fight-or-flight response to kick in. These could caase the spell casters to cast more mana than neccessary for the spell and thus supercharge it.
The haste, the surprise, the two binding spells, the fear… a lot of thing actually goes wrong.
Just my theory, but that is what I get from that situation.