See here
https://www.pecj.co.jp/en/fuse/outline/p3.html
An automotive fuse will blow if it is subjected to 15 amps continuously for a 100 hours. That is it rating. (Radios wont do that)
If you want it to blow in instantaneously (0.02 sec) , you need a current 600% higher than its rating.
There are may different fuses so the figures my not be exact but the principle is there.
Sometimes the inrush current when switching a device on can be many times higher than operational. However most amps these days have soft starts. If you are old enough, you will remember speakers giving out a huge thud when the amp was turned on. (My old hifi at home actually dipped the ceiling lights when it was turned on)
I'll put my money on short.
On constructive note, if the fuse wire is splattered all over the inside of the fuse holder, definitively a short. This may not always be true, if the wires from the battery are not that large, (as in my 986) the resistance in them may not allow the large current flow, so it will just blow it 'normally'