I don't know a lot about sps - but I keep them with more success than failure. Every time I've hesitated to frag or glue, I've regretted it.
I kinda don't buy into the too clean theory - by my logic, if being too clean was the cause, the glue couldn't stop it - but it does.
I suspect a bug, germ, bacteria, amoeba which attacks from the base - probably something normally living on the rock or plug - which blooms and spreads to the base of the coral where it feeds on tissue and works its way up the structure. Whatever it is - it can't easily cross over or under the glue barrier.
In my tank - I tend to associate stn with a sharp alk or salinity swing. I run a mixed reef with lots of fish and LPS - clean is not my problem.