Do you have your grocery store saved as one of your favorites?
That would explain it then as Maps tends to default to either "home" or "Work" or other "saved" destinations.
Knowing that you were not heading "Home" as indicated (like taking a turn in a completely different direction) it will suggest another destination and give you the option to select "go"...but will eventually just route there on its own after a few seconds if you don't select "go" yourself.
The automatic suggestions are scary based on the time I leave my house...I didn't realize I was so predictable...