It boils down to a chain of errors.
- Archbishop Lefebvre was automatically excommunicated “by the law itself”* the moment he ordained anyone a bishop; if he’d done only one, it might have been lifted formally immediately, since he’d been promised that he’d be granted permission to ordain one bishop. Likewise, all four of those bishops were excommunicated By the law itself* by accepting their illicit ordinations
- having been excommunicated, he became inelligible for leading the SSPX; they should have immediately called for (by canon law) a conclave to elect a new superior general or had the pope appoint a locum tenens.
3a) after the recognitio of the excomunication was published, they reelected ABp. Lefebvre, who was by the law ineligible to be (re-)elected a superior general, as he was under excommunication.
3b) After ABp. Lefebvre passed away, they elected another excommunicated man Superior General: Bishop Fellay.
3c) They have continued to reelect Bishop Fellay. He only became elligible when the excommunication was lifted. Even then, were not the entirety of the society suspended, he’d be ineligible due to the suspension!
- They have as a society ignored a papal suspension. Itself an excomunicable behavior (but one that requires recognitio, and isn’t “by the law itself”*)
Note that there are those who claim the SSPX is not punishable under the 83 CIC… but the punishments and requirements for these issues are exactly the same as the requirements under the 1917 CIC.
Whether or not the initial excommunication was good or bad for the church as a whole, it was proper by the extant traditions of canon law, and by both the 1917 and 1983 Codes of Canon Law (CIC).
- By the law itself - “laetae sentencae” - some actions render one automatically excommunicated without that excommunication having to be issued by a superior. The formal issuance of the public notice is the point where it’s being made public so that they might feel pressured to repent their errors.