Im guessing the fish wasnt doing well and probably floated dead or near dead into it, I feed some of my larger shrooms 3 to 5 '' across every few days and they close so slow they wouldnt be able to hold onto something healthy. Even my big Anemone's I dont think could take a tang that big unless it was damn near dead.
But it makes for a good vid
Edit: Look at the tail of the tang its almost bone.. seems like it was dead before it got into the Mushroom IMO.