erc-track-switch-direction

erc-track-switch-direction is a variable defined in `erc-track.el'.
Its value is

oldest

Documentation:
Direction `erc-track-switch-buffer' should switch.

importance - find buffer with the most important message
oldest - find oldest active buffer
newest - find newest active buffer
leastactive - find buffer with least unseen messages
mostactive - find buffer with most unseen messages.

If set to 'importance, the importance is determined by position
in `erc-track-faces-priority-list', where first is most
important.

You can customize this variable.