Aha. /etc/NetworkManager/dispatcher.d/01ifupdown run-partses /etc/network/if-up.d/ on "up" and /etc/network/if-post-down.d/ on "down" (which is actually "post-down" in ifupdown terminology). And there is no /etc/network/if-post-down.d/bind9 so named doesn't get nudged when NM takes down an interface. Just reported this in bug #1087228.
Aha. /etc/NetworkMan ager/dispatcher .d/01ifupdown run-partses /etc/network/ if-up.d/ on "up" and /etc/network/ if-post- down.d/ on "down" (which is actually "post-down" in ifupdown terminology). And there is no /etc/network/ if-post- down.d/ bind9 so named doesn't get nudged when NM takes down an interface. Just reported this in bug #1087228.