Speedometer cable question. Failure.

[Vanspeed]

In my attempts to put my fuel system back together in my f-1 I noticed something. While I was underneath routing the new fuel line, I noticed my brand new speedometer cable was almost in a knot. I quickly went to remove it from the trans and the end is completely twisted off. What would have caused this? I got the cable from macs and it seemed very smooth. I tightened both ends snugg at the speedo and trans. The cable looks like it was spinning with the internal shaft until it ripped off??