From what I have read it to kind of load balance their system. If a channel is becoming congested they may switch channels automatically to improve performance across all channels of the network.
some WiFi Routers/ AP’s do change the channel without rebooting. Some reasons include, but not limit to:
- Noisy environment on specific channel
- Lots of other AP’s on the same channel
- Interference with Bluetooth and/or other RF Communication
- Intrusion Prevention/Detection Systems trigger such as routers sensing that the WPS PIN is being bruteforced by Reaver or other tools
- Another AP with the same ESSID (name) on that channel
Please note that this behavior is very manufacturer specific / firmware specific, for example some Open Source Firmware such as OpenWRT or DD-WRT can be configured to hop channels based on user defined conditions