Why?

Why is this phenomenon limited to dogs?

Why has this never been noticed previously?

Also, why have I never observed this behavior with my dogs?

And to the comedian who replied as well: dogshit and bullshit are not the same thing.

I wouldn't be so quick to call BS. They state in the abstract that it's not limited to dogs:

Several mammalian species spontaneously align their body axis with respect to the Earth's magnetic field (MF) lines in diverse behavioral contexts.

You can read more at https://en.wikipedia.org/wiki/Magnetoception --- though, this is not without issues:

The largest issue affecting verification of an animal magnetic sense is that despite more than 40 years of work on magnetoception there has yet to be an identification of a sensory receptor.

I remember having read about this effect in homing pigeons, and the Wikipedia article also mentions that:

These results suggest that magnetite located in the beak of pigeons may be responsible for magnetoreception via trigeminal mediation. However, it has not been shown that the magnetite located in the beak of pigeons is capable of responding to a magnetic field with the Earth’s strength. Therefore the receptor responsible for magnetosensitivity in homing pigeons has not been cemented.

Without being an expert, it seems to me to be worthwhile research.

I don't think they're saying it is limited to dogs. It's just proven in dogs. Dogs behave really oddly when a natural disaster is about to occur, would it be that surprising that they had an unique sense that us humans don't have such as what this study is suggesting?

Maybe I can hire an augur to read my dogs poops then play the stock market with the findings?

you mean your dogshit detector?