A means of survival
According to Katerina Harvati-Papatheodorou from the University of Tübingen, other factors may be at play when it comes to interspecies breeding among the hominins. According to her theory, it could be a means of making the species survive.
Source: Atlantic Mirror
Most ancient human groups were small in size and the mortality rate was drastically short. They may be aware of other groups dying out at some point. Breeding with another group may be seen as a means to reproduce and grow in numbers.