how does threading get disabled?

[travis+ml-mutt]

Every once in a while I find that email is no longer shown as threaded,
and I can't figure out how to turn it back on.  Hitting esc-V makes it
say that threading is disabled.  Any help?
-- 
https://www.subspacefield.org/~travis/
I need a better strategy for being less analytical.
For a good time on my email blacklist, email [EMAIL PROTECTED]