The traditional switch from Sie to du is called Brüderschaft trinken, drinking brotherhood. The dictionary DWDS explains it as agreeing to say du to each other, and records phrases such as wir haben auf Brüderschaft angestoßen, we drank a toast to brotherhood, and wollen wir nicht Brüderschaft machen?, shall we not drink brotherhood? German Wikipedia notes that younger people and workforces usually switch to du at once. If the du is offered from the wrong side, older or more traditional people sometimes find it inappropriate.