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.
It's not an unusual cause of explosive eruptions, but Mt. St. Helens is ~50 miles from the ocean. Magma can build up pressure entirely independently of seawater-induced steam explosions.
IIR, the Mt. St. Helens 1980 eruption mechanism was the slow build-up of a steep slope of very weak rock, while various earthquakes (associated with deep magma movements) jiggled the whole mountain. The little glaciers on top were, at best, along for the ride when the "final" 'quake turned the unstable slope into a big landslide. Which landslide also "uncorked the champagne bottle" for the main eruption.
> "When you put a ton of seawater into a cubic kilometer of liquid rock, things are going to get bad fast," he says.
Seems like a catastrophic failure.
The Big Island of Hawaii constantly has lava and sea water mixing on small scales. Makes me wonder if it’s at risk of a subterranean breach as well.
It seems like this Tonga explosion took nearly everyone by surprise.