The honest answer is still "who the hell knows for sure". It could have been sunk by either an internal coal fire leading to explosion or external mine or even IED. But still even 120 years on there has not been conclusive evidence that has come to light one way or another.

You may be interested in this old thread that goes into the various theories, the series of investigations, and their differing findings. https://old.reddit.com/r/AskHistorians/comments/2kma0f/in_my_high_school_history_classes_the_fate_of_the/

My personal stance remains that if the 1970's investigation satisfied no less than Admiral Rickover it was most likely a coal fire, thats hard to argue with.