Technically - your wireless webcam shouldn't cause that type of network activity - but it really depends on how your network is built.
Most home networks are built around some sort of consumer router which will typically include a four port network switch and maybe a wireless access point as well - network switches are supposed to keep network traffic separate so the traffic between the webcam and the PC should not reach the NAS and vice versa.
If your network is built using a hub rather than a switch, which could be possible if you have an older model router, then all the traffic would reach all the devices, and you would see activity in places you might not expect.
Most network traffic is sent as "unicast" traffic, "uni" because it is intended for only one recipient, but some may be sent as broadcast, intended for all recipients - excessive levels of broadcast traffic, will show as activity in unexepected places and may be a sign of a larger problem eslewhere.
The trick is in knowing what is expected and what is not - and the only way to really determine what you are seeing is with a network analyzer.