This is probably more than you want to know about it but explains the technical aspects. Basically, SSPX doesn’t agree with Rome on the current path of the Catholic Church and hasn’t since Vatican II in the late 60’s. While they aren’t alone they have openly disagreed with Rome and flaunted it by consecrating Bishops etc. They have been excommunicated before and had it rescinded in a spirit of reconciliation but have refused to participate in any dialogue. I don’t have any specific knowledge of the situation but have read some articles about it recently. IMHO, if they feel that strongly then they need to leave the Catholic Church and start their own religion instead of trying to wag the dog. 

https://www.catholicregister.org/item/4111-vatican-declares-sspx-in-schism