To be clear, I'm happy that it doesn't when my cron job alarm clock goes off and the headphones have been pushed off my head when I went to sleep streaming audio from youtube, but it is inconsistent.
How can I fix it?
Edit for clarity: It's not a cron environment problem. It also happens if I open an xterm and launch it that way.
This was fixed in 16.04 at least on my box.
The catch is, pavucontrol now remembers which app has which default sound sink and which fallback sound sink. So, I can choose to set my USB headphones as an out and the on-board as a fallback, so they get used if the headphones get disconnected (I think it's the other way around actually - USB specified as a fallback, but they kick in when plugged in), but it's app specific.