Human resources development is connected to adult learning because much of human resources development consists of helping adults to learn.
Human resources development generally consists of two main types of activities. On the one hand, there are activities that are meant to improve the organizations. They are meant to help organizations structure themselves in ways that promote learning on the part of their employees. On the other hand, there are activities that are meant to promote learning on the part of the individual employees. These activities typically involve various sorts of training.
The connection between this second aspect of human resources development and adult learning is clear. In this aspect of human resources development, workers are trying to learn new skills. Presumably, almost all workers who take part in such programs will be adults. Since they are adults, and since they are trying to learn new things, their activities are connected to adult learning.