[kroeckx]

For music copyright is split in 2 types: composition and recording. The copyright on the composition is probably expired, but on a recording most likely isn't. The automated system most likely claims it's a copy of a copyrighted recording. Since it's a popular piece, it can be hard to tell recordings apart.

Note that the composition might also still have a copyright because it's not the original composition but a derived work, but that seems unlikely in this case.