Geoff,

They should not be related events at all.
I think you may need to look a little closer at
the Domino server log to find the culprit.

For TDP for Domino...
If the databases are logged databases, they will not be
backed up by TDP for Domino "incremental" unless the
database instance ID has changed. Things like
compacts and/or other maintenance may change the
database instance ID.
If the databases are not logged, they will not be
backed up by TDP for Domino "incremental" unless
the "meta data" date or the "data data" date changes.
(These dates are stored inside the .NSF file itself and
accessed via Domino API calls.)