OpenDialog causes odd floppy disk drive activity

I use the OpenDialog object in Delphi 1 to get the name of a file from
which some reading is done.
The file will typically be on a diskette in drive A:.
After the dialog is closed, the reading is finished and the routine in
question is closed, the program will go on with completely other tasks,
like opening, closing, and working with some tables.

Then, strange things happen. The diskette drive will start buzzing now and
then, for no purpose, considerably slowing down activities.

Obviously, A:  has become the operating system's default drive.
Why? How to reset default drive to C: ? And why is the Delphi program
"checking" the default disk anyway? I don't ask for that in my code... When
I open a table, I know where it is and it is never on A:.

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