The explosion, say Pavel Jungwirth and his collaborators at the Czech Academy of Sciences in Prague, is not merely a consequence of the ignition of the hydrogen gas that the alkali metals release from water. That may happen eventually, but it begins as something far stranger: a rapid exodus of electrons followed by explosion of the metal driven by electrical repulsion.
Neat. It’s not the hydrogen reacting with air that causes the alkali to explode. That reaction doesn’t cause more surface area to be created as the reaction unfolds, so it can’t “accelerate”
Video in the link, including slow-motion views of the explosion.