As far as I can tell (from experience and light reading), performers can copyright their performance of public domain works (or works that are copyrighted to other people, but obviously not the original work which is either public domain or belonging to whoeverholds the copyright). So what I'm guessing is that the thing picked up that your composition sounds similar to the performance of the same piece by whoever that is and is assuming a copyright violation.

Which seems kinda dumb to me but copyright law is convoluted and weird. Creative commons is much more straightforward XD