Pobi and Pubo felt that their Boss was simply pretending to be a hostage just as a test to see if they were loyal to him or not and whether they would try to save him.
The fact that Brono might really be in trouble didn't really settle in their minds since even if the strongest weapon was handed over to a rabbitman, there was no way their boss could get overpowered. They just refused to believe that.
So they glanced at each other, creating an understanding with each other that this was their opportunity to show their boss how useful they could be to him.
By thinking that, they reached their hands towards a firebomb in the shape of a ball, they had saved up in case they ran out of arrows.
Brono was someone with a sharp eye and saw that the twins Pobi and Pubo were up to something, and Brono, knowing them, realized what they were going to do, making him shout hysterically, "You IDIOTS! DON'T!"
