Based on my own experience, these worms spawn every X amount of time, although it is said that they spawn faster if you keep attacking the boss.
I went to this boss the first time in a 4 players team, and at the end of the time, we had like 5 Hunger Worms around in the respawn upstairs; and I can assure you that every time we were upstairs, even in the beginning, when a Hunger worm spawned we focused on him, but as we had not enough damage to kill it fast, another Hunger worm spawned while the first one was still alive (and we were not even attacking the boss).