Interference patterns fighting over the frequencies available. If your mouse is at 2.4Ghz and the Wifi is at 2.4Ghz: due to one device not supporting 5Ghz like an older access point [even though it supports like the Airport Extreme Base Station 5Ghz] the timing between the two devices using the same mesh frequency will interfere with one another.
Ideally, if all wireless points are at 5Ghz your mouse should never interfere.
Magic Mouse uses: http://www.digchip.com/datasheets/parts/datasheet/072/BCM2042.php
Could well be that then. My Router is BT Home Hub 3 which i don't think is dual band. We've been having issues on all computers with the wireless randomly slowing.