2015-10-21 Update: Version 2.1 of tmux has just been release a couple of days ago and from what I can tell it has addressed this fatal issue! So the simpler solution would be doing brew upgrade tmux to get the recent release. If you are stuck using 2.0 however, I hope this post is helpful.

What fixed it for me was adding a single line to my ~/.zshrc and a single line to my ~/.tmux.conf file:

# ~/.zshrc
tmux start-server

# ~/.tmux.conf
new-session

It turns out, after trial and error, that the OS freezes (or in worst cases, kernel panics on older OS X versions) usually happen when you close the very last of all tmux pane of the very last tmux session.

With these additions to my config files, a tmux server is started with an empty tmux session whenever I fire up a terminal window for the first time. This means that I always have an empty tmux session – still active although never used – even after I exit out all the sessions I create on a daily basis.

I never had my MacBook Pro grind to a stop as soon as I close out all tmux sessions and having to forcibly turn it off.

This does not fix tmux, but is a simple work-around for a problem that has a potential to incur data loss in the future.