Hobbes is quite aware of the fact that, by his definition, beasts will actions as much as human beings do, because they have desires every bit as much as human beings do. That is, it fits the phenomena it is designed for and is explanatorily superior to the ordinary way of thinking of them. It is metaphysically economical in that it does not commit him to any entity, such as a mind or soul, other than bodies and it coheres with his definitions of other cognitive and affective terms. Hobbes's analysis of will is part of his attempt to give a thoroughly naturalistic and mechanistic account of human psychology.
Wills are desires, and desires are immeasurably small motions of bodies.
As a motion, instances of will are caused by other motions over which the agent has no control. An agent does what it wills but does not will what it wills. But it is absurd to hold that a power or faculty causes anything.