Sheesh, how does that even happen? Does Hg just spontaneously appear?

Sorry, can't watch video at work.

Farmed salmon usually has fish meal (sourced from wild-caught fish that has lower economic value) as a significant part of its diet. Those wild-caught fish concentrate organomercury compounds like other wild fish, and the compounds pass on to farmed salmon who eat them.

I haven't watched it in a few months, but salmon farming happens in net-enclosed partitions of natural bodies of water that are polluted with industrial waste. Pesticides are sprayed directly into the farms in order to combat disease. The pollution from the farms themselves makes the situation even worse as time goes on.

Here's a resource[1] for anyone who is curious but doesn't want to sit through a YouTube video.

[1] https://livingoceans.org/initiatives/salmon-farming