When a species grows unusually fast, there isn't enough balance between birth and death. This means that there is more competition between members of this species and/or competition between this species and others.
Rapid population growth is never good. Typically this leads to higher demand for survival essentials such as food and living space in a species. The demands for food can drive prey extinct and backfire on the growing species. This can lead to mass death and possible extinction for a over populating species
