Transit times suddenly fast

I have sold on Amazon for a number of years. I’ve had automated shipping settings on for several months. Suddenly in the last week I’ve had a few orders come through with transit times of only 2 days and my only option is to use 2nd day air. I’ve contacted Amazon to ask why I’ve having to use expedited services when then buyer elects standard shipping but they just say this is the way automated shipping settings are designed. Makes no sense to me. I can’t continue to have to ship certain orders at this expense. Anyone else experience this?