An operation of law is when the law dictates that a person has certain rights or obligations that they did not ask for, which resulted from laws already on the statute books.
A common example of an operation of law is when a person dies without a will and their beneficiaries are determined by the regulations set forth in law.
Another operation of law is the requirement on a homeowner to pay property taxes related to that property. Since they own the property and the law dictates that taxes must be paid on property by the owners, by default any property owner owes taxes as an operation of law.