I always thought it was a sizable meteor that broke through the mantle, creating a supervolcano, that continued the process. The fissures got larger with the mega eruption, additional volcanos were created, and the fissures spread. The new plates began to drift away from the impact site, which eventually solidified over time.

(I guess it was because the motion remined me of marbles packed together, drop a marble on them and they scatter in all directions...)