Jukeboxes emit a redstone signal while they are playing a disc, and they stop emitting the signal as soon as the disc ends. This redstone signal blocks the hopper from taking the disc before the song has ended. Hopper minecarts can only be disabled/enabled by activator rails, so they are unaffected by the redstone signal from the jukebox. But if you put a hopper minecart under the jukebox, the hopper minecart cannot be correctly taken out of the record.