Does anyone know the exact process of marriage by default?
There is no such thing as "marriage by default." That phrase would indicate that a person ends up married by default, i.e. through no action on his/her part, and that's not possible.
Does it have anything to do with either party just not doing anything at all to help the process of divorce?
There may be an award of divorce by default, i.e. one spouse files for divorce and the other spouse never responds and never contests the divorce, then the spouse that filed might ask the court to go ahead and grant the divorce based on the other spouse's failure to respond to the divorce action.