Ever since my near-brush with movie fame (they finally contacted me and decided to go with a more amenable engineer, like the Niagara Tunnel!), I have been struggling with the cyclic nature of volcanoes. They tend to operate on the same sort of cycles as earthquakes. Some may think they get magically injected with booster juice every 400 years, but I think it is standard rock mechanics.
Start with a volcano blowing up. Has it used every speck of available magma? No, there is always a huge magma pool, and we puny humans only get a spitball. The magma relieves the pressure and then sits back in deep reservoirs. But the magma is much less dense that the surrounding cold rock, so it is actually the rock that is the supporting container. This support can go on for 400 years, but if that darn volcano keeps spitting out ash, it creates this huge weight that is like piling on elephants at the circus! Eventually, the bottom elephant has to collapse, and this squeezes out the next explosive spitball. KABOOM!
If there is a large magma pool and the rock blows open under the sea, then we get the giant boiler explosion. About 70,000 years ago they had such an explosion, that gave a nuclear winter for 10 years, and cut the world human population to just 10,000. That's right beside this volcano. Makes you think.