I noticed that on some take up spools, the standard metal type, the film is trapped by a sprocket hole on a "tooth" on the bobbin. Rewinding past this point will rip the film off the spool or tear as you have found. The way forward may be to rewind to the point where it stops and the remove the film cartridge and spool to release the rest. There should be no problem with this as this is the way that it was loaded. I guess that the skill is in stopping at the right point.
As a note, I have found odd bits of film inside some FEDs, once lodged in the curtain track, causing shutter problems. I think that they got there when the film was rewound off the bobbin, tearing small pieces off.