Is it acceptable to leave an adjunct teaching position 10 days before classes begin?
I think it's generally agreed that it is unwise to tell an employer that you might leave, that nobody does this, and that nobody expects anyone to.  You only tell them when you are definitely leaving, which you did as soon as you knew.  You haven't done anything inappropriate.
Your old department head is surely not thrilled about having to fill your classes with 10 days to go, but it's their job to handle such contingencies, just as if you had been hit by the proverbial bus. You weren't under contract and so you're a free agent. I probably wouldn't expect them to hire you as an adjunct again, but it seems unlikely you'd want them to anyway.