While this is true in that the specific thing that has excommunicated them and made them schismatic, the reason they want to do that is 100% about Vatican II and about the popes being horrible and etc. Marcel Lefebvre, the founder of the society in question, founded the Society of Saint Pius X because he was very very very upset about things like religious liberty (the idea that believers in other Christianities were not inherently heathens, heretics or apostates just because they weren't Catholic, among other things). His entire purpose was a clear, vehement rejection of Vatican II, although he did tend to at least in public be circumspect about dissing the pope. At the same time, he was very clear about perceiving the entire enterprise as an error that the papacy was allowing, and his society was named after Pius X for a reason.
Shooting his mouth off about this got him told that his Society should in fact cease to be a society . . . which he refused to obey and he refused to disband it. The Church told him to do as he was told and told him that until he did as he was told he was not allowed to lead Mass or administer sacraments. He . . . . continued to do that.
Marcel Lefebvre started just ordaining priests - ones that he had trained, of course, and that shared his point of view - in spite of not having the blessing of his bishop and also being told, directly by the Vatican not to do that. He was almost hitting the end of his rope when Paul VI died, and despite his being a dick about things, John Paul I and II both tried very hard to treat him with kid gloves.
He was excommunicated (along with the rest of the Society) the first time (because remember this is the second time) in 1987, because he was dying and he wanted to make sure there was a bishop for the Society that toed his line, and he didn't have one handy. He did, and the act automatically excommunicated him. The Lefebvrists remained excommunicated until Benedict was a total marshmallow at them; to their dismay, Francis didn't like them as much and started informing them ...that they had to stop pushing their goddamn luck about getting in line with Vatican II.
The bishops they ordained to excommunicate themselves again this time? The reason they couldn't get papal approval for those bishops is because those bishops are also anti-Vatican II and make no bones about it and would continue the Lefebvrist line of insisting that they're a modernist, Freemason-born blight on the face of the Church, especially the parts about not everyone going to Hell if they're not Catholics and especially especially the parts about the Jews not being the murderers of Christ.
It is all, always, about Vatican II. Vatican II is the reason that the Lefebvrists exist as an organized, identifiable thing; Vatican II and their rejection of it is the reason that Lefebvre himself needed to do the ΓcΓ΄ne consecrations that got him excommunicated in the first place in order to make sure that a bishop who rejected Vatican II existed in order to leave his order to them, and it is absolutely 100% about needing bishops who held and hold the same stance on the reforms of Vatican II - and the papacy saying "No, you DON'T get to ordain those ones, they are NOT suitable, try again" - that meant the Lefebvrists went through with this set of consecrations, thereby automatically excommunicating themselves a second time.
Yes, the element of action that causes the automatic (no declaration by Papacy even needed!) excommunication is, indeed, that consecration.
But the reason they were doing it?
Is absolutely, 100% because they reject Vatican II and think it's a horrible blight on the Church that it happened and that it has not been rejected/repealed/recanted.
And it is important to keep in mind that they are doing so as a tiny fucking minority because even within the Church with all its flaws, nearly 3000 bishops got together, worked for multiple years, and overwhelmingly - always overwhelmingly, often at rates of 97%-3% - voted in favour of enacting them.
But it is always about Vatican II. If it weren't for their rejection of Vatican II they would never have had their candidates for bishops rejected, and it wouldn't be an issue!