The subject may still have the semantic role of Instrument, that is, the entity (generally inanimate) which the Agent uses to perform an action.
Yī kuài shítou fēi lái ,dǎ pò le tā de tóu.
(A stone flew over and broke his head.)
Nà liàng qìchē bǎ háizi zhuàng dǎo le.
(That car knocked the child down.)
The subject NPs in (1) are inanimate entities causally involved in the events. They exert no action of their own; instead, they must be acted on by something else in order to participate in the events, so the energy source is external to them. Thus the responsibility for the events is ultimately attributable to humans. Hence the subject NPs are Instrument, which is closely associated with Agent.
 Loar, J. K. (2011). Chinese syntactic grammar: functional and conceptual principles. New York: Peter Lang.