According to new scientific research, humans are hardwired to perceive as agents anything that "moves by itself." We do this from when we are less than 1 year old. Babies think of humans, animals, and computer simulations of bouncing balls as agents; if there is no readily-apparent external cause, we say that the object has agency, and assume it is an agent.
But nowhere in my science readings have I found a good explanation of agency in biology/physiology. The best I have seen are attempts to bootstrap quantum notions of chance to the human (+emphasis) brain in order to refute determinism.
Not suggesting that, like bouncing ball simulations that can be understood in terms of programming, you can explain complex animal behavior with DNA or genetics alone, or ever really explain it in terms of causality. But perhaps agency and free will/choice/volition/intentionality are not necessarily bedfellows.
Perhaps an agent is an object that is very good at concealing its cause, and so to humans--who are wired to perceive causality--appears to have no cause and therefore we say it has volition.
Maybe I've been reading too much Object Oriented Ontology (OOO), or too much early Foucault, or I dunno what.