A bandwidth saturating download is not "simple" in network terms. TCP based networking is packet based with an acknowledgment, and every usually ~1500 bytes of data is an individual packet. Every single individual packet over wireless has to be sent and confirmed received before the next one can be sent, so if you are talking about a 600kbps download (easy even for pokey broadband speeds in the US), thats ~400 packets a second that have to be sent with ~400 confirmations that have to be received. If any one of those packets gets blasted by interference, thats another round of transmit and acknowledgment, holding up the entire "stream". Mix in other TCP based traffic on the machine, along with wireless beaconing and other overhead, and you have a lot of individual transactions going on over WiFi. Then there is your bluetooth mouse, that based on the spec, can change frequency 1600 times a second, requiring 3200 exchanges in one second between the bluetooth radio and the mouse.

Dig out a 10mbit hub, and see how often the collision light blinks just based on casual web browsing with 2 machines. Toss in a file download on one, and the collision light will be on quite a bit. WiFi is like a hub, but with more overhead when multiple devices are connected due to all the wireless negotiation and tracking.