It looks to me that the excommunication was for continuing to say public masses after he resigned. The excommunication was not because he was dissident wrt Church doctrine. I wonder whether anything would have happened if he had not resigned.
The hierarchical authorities generally rarely or never explain exactly what a laicization or an excommunication was all about, and this can be a problem. Years back (I'm talking 20+ years ago) I remember a Jesuit, Bill Callahan, who was evidently laicized and likewise expelled from the SJ's for advocating women's ordination in an intemperate blast in the New York Times. He, however, portrayed his ouster to his friends and allies as a punishment for "opposing the arms race and advocating for social justice for the poor", which is nonsense because the SJ's and in fact the Holy See were all about peace and poverty issues.
But it's always handy to use the poor as human shields.
"The" "issue" in his case was women's ordination. That, plus public defiance to his lawful superiors.
My impression is that messing with sacraments, plus open insubordination, will get you in deep doodoo every time.