Once in a while, the beasts in the neighboring forest congregate and attack Reinfield City. They do so to eat the humans, eat the cattle, and gain access to the energy springs in Reinfield City.
Humans are not the only ones who need energy springs to survive, and undying godclads are not the only ones who need to drain or fuse with an energy spring in order to advance.
So not only are small and medium energy springs attractive to beasts who just want to live and survive, but the large energy spring at the center of the city is also attractive to the powerful beasts who want to consume it to advance.
Once in a while a powerful beast reaches rank 6 and becomes ambitious. The foolish ones attack Reinfield City and are quickly put down. But the smart ones wait until there are many other rank 6 beasts to attack with.
The rank 6 beasts also rile up the beasts under their control and in the forest to attack Reinfield City. This is usually what causes the beast tide.