Ugly bastard is not only a fuel, but also an oxidizer, so you only need enough heat to cause it to combust. It’s so volatile that only a small amount of heat (60 Celsius) will cause a chain reaction, making it burn at temperatures exceeding 10,000 Celsius with very small amounts. The true cause of the initial reaction is unknown, due to difficulty observing. The most popular theory is the energy coming from the combined rage of every watcher suddenly seeing the ugly bastard when they didn’t expect it.