Unexpected Delete

I have a table with many existing records, and it uses an autonumber key. I
recently added another table for additional information that only applies to
some, not all, of the records in the first table. These tables have a
one-to-one relationship, and the second table's primary key is supposed to
match the first table's primary key.

I add records to the second table by selecting from the first table via a
combo box.

My question is: What could cause Access to delete the record from the first
table just because I delete the record in the second table?

Directional: I have heard the term "directional" applied to relationships,
but none of my reference books breathe a mumbling word on this topic.

Referential Integrity: At first I had NOT enforced referential integrity, so
in desperation I tried enforcing it, but it made no difference.

Thanks,
OldBlindPew