If it's ok again it as almost certainly preignition ie the tip of the spark plug getting so hot it sets the fuel exploding before it actually sparks. My old Norton is prone to this. You just switch to the next heat range colder plug when it's really hot, problem goes away. It can sound alarming , as if the big end has gone due to the fuel explosion trying to force the piston back down before it's reached the top, known as "knocking".