Mob Respawn Times
Some areas are set up to add new mobs at regular intervals, so if one is slow clearing the first “wave,” one would have to deal with multiple waves at a time.
Like you, I found it frustrating at first. But then I decided I should try to improve my build/gear/tactics and I’m happy that the game “encouraged” me to do this.
In other words, I suspect this is intended by design and not a bug.
John Smith: “you should kill monsters, because killing monsters is awesome.”