Really curious.

Anything of note when you look at the "cut" ends? It doesn't look like a clean cut from the photos, if it's been caused by trauma it looks more like a it was trapped, spine snapped and then the flesh torn. But that is pure speculation, there would need to be something for it to have got trapped by.
How does the lost bit of tail look, any sign of necrosis? It may be difficult to tell if it has been detached for a day or so.

15-20cm of tail lost... I'm staggered.