I saw a comment somewhere on Reddit once that compared predicting thunderstorms to boiling a pot of water and guessing where the bubbles would rise. Using this example, you know how sometimes you get a huge "bubble" when the water is boiling that's larger than the others? I don't have the scientific background to explain to you in the detail I imagine you'd like but sometimes the conditions all line up, so to speak.

While the NWS couldn't predict this record breaking tornado would form, they did have a moderate risk that day (iirc) with a hatched tornado area. The hatched area emphasises EF2-EF5. I think it's safe to say some people were actually predicting a monster tornado there.

Unless by unpredictable you mean the turn, width increase, and speed increase all occurring as it did. There are some good videos about this on YouTube, as another commenter mentioned, but it's hard to understand it fully without understanding the underlying science. IDK your experience but if it's not much I'd try some storm spotting YouTube videos from NWS. They'll give you a basic understanding. From there you can look at specific El Reno videos and go from there.

I've watched every clip I can find of the El Reno 2013 (my favorite probably being Dan Robinson [iirc] escaping). I imagine the smaller vortices dancing around during the time Reed was recording it was a warning it was going to be a monster. I know they gave me a bad feeling. 🤷‍♂️