So, just as there are two paradigms regarding what happened to the Church:
1) many Churchmen over time had their minds contaminated with Modernism eventually got into higher offices and want to spread their "truth"
2) enemies who hated the Church infiltrated with the deliberate intention of detroying it
Same can be said of the SSPX. You certainly have some cradle-+Fellayites who grew up in SSPX and were trained at the seminary under the nouveau régime, but I believe that some of the old-timers are agents, infiltrators, and/or compromised. I've long had Schmidberger toward the top of my list of suspects.
Of course, you need both, right? If enemies had infiltrate the Church/SSPX, unless fertile ground had already been prepared for acceptance of their programme, they would simply have been rejected by the Church/SSPX, kicked out, leaving the Church unharmed.
I think Bishop Williamson leaned toward option one, where he constantly spoke of the Churchmen losing their Catholic senses. I don't believe that. I believe that the leaders were infiltrators, agentes, and/or compromised (blackmailed), so that the destruction was quite deliberate, but not some kind of grass roots development over time.