What I don't understand is, how exactly did it derail? Roller coaster trains have so many wheels on them (upstops); they're literally bolted to the track. Did something break, snap or come loose?
Knowing that Big Thunder Mountain can suddenly derail doesn't give me much confidence in riding it in the future. I know these things are rare, but there's nothing particularly fun about being harnessed/trapped in a roller coaster car, that can suddenly leave the track!