here is a useful link regarding it's dependencies:
Dependencies
What service Windows Image Acquisition (WIA) needs to function properly:
Remote Procedure Call (RPC) (S, HB, HP, P, U, E)
DCOM Server Process Launcher (S, HB, HP, P, U, E)
RPC Endpoint Mapper (S, HB, HP, P, U, E)
Shell Hardware Detection (S, HB, HP, P, U, E)
Remote Procedure Call (RPC) (S, HB, HP, P, U, E)
DCOM Server Process Launcher (S, HB, HP, P, U, E)
RPC Endpoint Mapper (S, HB, HP, P, U, E)
What other service require Windows Image Acquisition (WIA) to function
properly: None (S, HB, HP, P, U, E)
Which doesn't really help diagnose what's causing it, but at least shows nothing else should be calling it (ie no dependencies).
There are several security vulnerabilites which Microsoft have patched with it, so make sure you've got all security updates... Failing that i would be inclined to stop the service one night and see if it has restarted itself before the morning then check/change it's startup properties (mine is running here and set to automatic by default). Then again i like breaking things lol ;)