This is a good question, and I don't know the answer. But it points out the courage of priests like Fr. Zigrang and Fr. Laurence Smith and Fr. Stephen Somerville. They have sacrificed everything, given up the earthly comforts of the established diocesan situation (and those can be remarkably comfortable indeed), and "sold everything they had to come follow Christ."
There can be no possible earthly motivation to do so. There is no monetary profit, no earthly rewards, no prestige or even recognition. All they can look forward to is persecution and slander. Thank God there are a few loyal men like them who put the Truth above all earthly considerations.
Although he is not a traditionalist, one might put a priest like Fr. Haley of Arlington VA in the same category. He has sacrificed everything, he was persecuted relentlessly, he was even placed in a mental hospital, all because he would not agree to say that black was white. Now the evil Bishop Loverde is holding a trial to have him forcibly laicized for the crime of exposing the unimaginable corruption that exists in the supposedly "conservative" diocese of Arlington.
Did Somerville join SSPX?
Haley certainly did not.
Are you attempting to compare their actions with flat-out schism?
Fr. Haley was ecclesiastically tried at St. Charles Borromeo Seminary in Philadelphia (Justin Rigali's Archdiocese and well away from Loverde's turf) and the judge was my bishop, Thomas Doran, who also serves on the Signatura, the Vatican's highest court. I certainly hope that the evidence justified acquittal and that he will be acquitted. I will trust my bishop's decision (not yet rendered) in any event unless and until it is overturned on appeal. I suspect that you will judge the matter by the results which is what is wrong with our secular court system.