While I don't know exactly why you got upgraded to Deluxe, you can downgrade using the Clear and Start Over feature. Just keep in mind that the different TurboTax product handle different tax situations and their associated forms.
For example, if you want to downgrade from Deluxe, and you want to itemize deductions, for example, you can clear and start over with a different edition, but depending on the information you enter, you may you'll be asked to upgrade to Deluxe.
Here is a short list of some common tax situations and the online products that now uniquely support them: