Unexpectedly prompted for parameters using parameters of form

I am experiencing some strange behaviour in Access 2007. I have a form with
2 nested subforms (sub1 and sub2). In sub2 I use a master/child link to a
field from sub1. The data in sub2 updates properly when I change records
from sub1 or from the main form.

On sub2 I have a drop down control which uses a query for the row source.
This query uses a drop down control from sub1 as criteria to limit the
choices.

The problem - I worked on the forms last night and everything appeared to be
fine. Today I started the application and when it opened and attempted to
display the data in sub2's drop down box, I was prompted to enter the
paremeter for the drop down box on sub1 (which is the criteria for the
rowsource).
When I opened the form in design view and then re-opened it the form view
everything started working again. Strangely, I didn't make any changes but
when I switched to form view I was asked to save the form.

Now that I was asked to save the form, I am not having the problem. The
issue is that this has happened on several occassions and I am concerned that
when I deliver the database to the client this issue will start happening to
the users.

Any ideas why this is and possible solutions?