tmux mouse mode is being disabled somehow during a session

Sometimes while I'm working, I notice that tmux is no longer responding properly to mouse controls (usually as I try to scroll back a pane using the mouse wheel and get iTerm2's own scrolling behaviour instead). What could be causing this? I'm using Karabiner to map Ctrl taps to Esc, so I suspect I'm "accidentally" tapping Ctrl (thereby emitting Esc) and then pressing some key or other that deactivates mouse mode.

I can restore mouse functionality by detaching and reattaching.

Any ideas?