I`ve always been under the assumption S.E.E was a product of light charges of slow powder...
That's mostly where you hear about it but apparently it can happen with "moderate" charges under certain circumstances. In the .243 example Kikkonen wrote "then new" Norma MRP. I'm guessing a range of loading data was being worked up when the test barrel exploded much to everyone's surprise. So surprised that the test was repeated to verify and one wouldn't risk such expensive equipment lightly. It's anecdotal and apparently an unusual case, but makes for a
BIG caution sign. Note that about everybody would consider MRP too slow to be useful for reduced-loading a .243 WCF.
I don't think there's a consensus on what causes S.E.E. It seems to happen mostly with light charges of slow for volume, relatively hard to ignite powder in big cases leaving a lot of air space. Jacketed bullets which would offer more resistance in entering the bore seem to exacerbate the problem.
I know only what I've read. It's not something that I want to personally experience and exercise an abundance of caution to avoid. Particularly since nobody can define the boundary at which S.E.E becomes a possibility.
