How would you describe an intentional accident, and how does that differ from a passive accident? I don't see a scenario where the distinction could even apply.
Perhaps that's the subtle humour behind this linguistic change?
"I broke your mug on accident", at the fifth word, the listener/reader thinks the other person is about to say "on purpose", but suddenly they say "on accident". Hence it's funny, reduces the tension about them breaking your mug.