"Are you Scottish?"

Another reason I don't like this question is the fact that I'm not Scottish. When I reply, "no, I'm not Scottish," the next question will be, "then why are you wearing a kilt?"

Which means I have to start explaining myself to a stranger. I don't think I should have to explain my choice of attire simply because someone is rude enough to ask.

Again, it depends on how the question is asked. If there is a demanding tone in the question, it's gloves on the ice time.
If someone is genuinely curious and interested, I'll happily talk to them about the joys and benefits of kilts.

I may come across as cranky. I may even be cranky. But it's only with people who are rude, intentionally insulting, or trying to bully me into dressing me to their standards.
I chat with dozens of people every day who ask me about my kilts. The overwhelming majority are polite, friendly and interested in kilts. It is so commonplace I rarely mention it. Just today, as I was waiting at that same bus stop, three large guys stopped behind me and one said, "Hey, check this guy out!" I was wearing a 6 yard black wool kilt.
I turned to face them and the one closest to me nodded his approval and respect. Another said, "Cool kilt, man."
I said, "Thanks," and they continued on their way.