Male guppies will eat small fish. The fry of guppies are smeared with something similar to yolk, and this substance can attract big fish to eat them. Therefore, newly born small fish are likely to be eaten by other big fish or hungry female fish. In addition, guppies may eat their own fry after reproduction due to hunger or being frightened.
To prevent this from happening, you can try to provide enough food for the guppies and keep their living environment quiet and stable.