Could someone confirm this:
Inversion here is happening because "op zondagen" is a subordinate clause. If it contained a verb it would be at the end of the clause. Because of the subordinate clause starting the sentence, the verb "werk" is moved to the front of the main clause.
It's not really a subordinate clause (clauses always contain verbs, otherwise they are not really clauses), but an adverbial phrase.
Whenever you begin a sentence with something other than the subject of the main clause (notice how here I say main clause and not sentence anymore -that's because a sentence can be formed by a main clause, or a main clause and a subordinate clause in whichever order so to speak, or by two or more main clauses that are coordinated) there's subject-verb inversion.