Learn Dutch: Omdat de lava over de helling stroomt, belooft het een ramp te worden. - Because the lava is flowing over the slope, it is likely to become a disaster.

In this grammar structure, beloven te is used figuratively to indicate that an outcome is highly probable based on current signs. While you can think of it literally as 'promises to become', translating it as 'is likely to' or 'looks set to' sounds much more natural in English.

Since a disaster is a negative event, couldn't we use dreigt (is at risk of) instead of belooft?

Yes, you could perfectly well say dreigt een ramp te worden. Both work for negative outcomes, but they highlight different things. Dreigt emphasizes the danger or looming threat of the situation, whereas belooft emphasizes the near-certainty of it: the flowing lava makes the disaster almost a sure bet.

Does the pronoun het refer back to de lava?

No. Because lava is a de-word, we would use hij or zij if we were referring directly back to it. In this sentence, het is a dummy subject, much like the 'it' in the English phrase 'it is likely to become a disaster.' It refers to the general unfolding situation.

Why does the main clause use the inverted word order belooft het?