Hi,
it looks like when unbound stops (e.g. because it's being upgraded),
dnssec-triggerd just dies, never to come back on its own.
e.g. from my logs on buster this morning:
Oct 12 10:08:18 tomate dnssec-triggerd[966]: [966] info: dnssec-trigger
0.13 stop
Oct 12 10:08:18 tomate dnssec-trigger-script[31627]: Recovering
/etc/resolv.conf...
2017-10-12 10:08:18 upgrade unbound:amd64 1.6.5-1 1.6.6-1
2017-10-12 10:08:18 status half-configured unbound:amd64 1.6.5-1
2017-10-12 10:08:22 status unpacked unbound:amd64 1.6.5-1
2017-10-12 10:08:22 status half-installed unbound:amd64 1.6.5-1
2017-10-12 10:08:22 status half-installed unbound:amd64 1.6.5-1
2017-10-12 10:08:23 status unpacked unbound:amd64 1.6.6-1
2017-10-12 10:08:23 status unpacked unbound:amd64 1.6.6-1
[...]
2017-10-12 10:08:46 configure unbound:amd64 1.6.6-1 <none>
2017-10-12 10:08:46 status unpacked unbound:amd64 1.6.6-1
2017-10-12 10:08:46 status unpacked unbound:amd64 1.6.6-1
2017-10-12 10:08:46 status unpacked unbound:amd64 1.6.6-1
2017-10-12 10:08:46 status unpacked unbound:amd64 1.6.6-1
2017-10-12 10:08:46 status unpacked unbound:amd64 1.6.6-1
2017-10-12 10:08:46 status unpacked unbound:amd64 1.6.6-1
2017-10-12 10:08:46 status unpacked unbound:amd64 1.6.6-1
2017-10-12 10:08:46 status unpacked unbound:amd64 1.6.6-1
2017-10-12 10:08:46 status half-configured unbound:amd64 1.6.6-1
2017-10-12 10:08:47 status installed unbound:amd64 1.6.6-1
Cheers,
Julien