To clarify: the timing of the explosion took everyone by surprise, not the fact that it exploded.

If there was forewarning I haven’t seen an article about it yet.

It had been erupting since December, including an explosive one two days earlier (https://www.rnz.co.nz/news/pacific/459572/underwater-volcano...) that prompted a tsunami alert. https://en.wikipedia.org/wiki/Hunga_Tonga#December_2021%E2%8...

There had been coverage of the eruption between December 29th and January 4th, and landslides due to the island's instability were certainly reported as a possible danger. Among other sources, this was covered extensively by the GeologyHub channel on YouTube which pretty much predicted this collapse.

Look up the GeologyHub channel on YouTube. He had been warning about the risks of landslides and the explosion risk they cause the day before.