If someone documents a natural disaster like a Volcano engulfing an entire city, that's not political in any way, that's just recording facts that any historian regardless of their political views would record.
There have been N volcano eruptions over the history of mankind, why you are taking focus on one and not the other is the political part.
The choice of which cities they document and what they cite is definitely political. For example, what city do you think of with that description? It's probably Pompeii and not Herculaneum, let alone Cuicuilco, Vestmannaeyjar, or NA 860.
So, what was the political motivation of Pliny the Elder to document the Vesuvian eruption that destroyed Pompeii, as opposed to e.g. the numerous eruptions of Krakatoa?