This doesn't explain the phenomenon but shows what you mean, I saw this storm myself, the first two strike were normal thunder and the third big strike was the shot gun type, infact it's still the loudest rumble I've ever heard and I was miles away, dread to think what it sounded like right under it. Although it's quite possible another one happened closer may be. Storm did everything, heavy rain, hail, snow, and damage.
http://www.ethiogrio.com/news/world-news/9728-lightning-bolt-halts-eurostar-and-parts-of-britain-are-still-covered-in-snow.html