Does there have to be a 'problem'? 30 minutes from pushback to airborn for a Classic is not at all unusual. Maybe they were doing a crossbleed start (using high pressure air from an already started on stand engine to start the others if the APU is u/s). It takes higher thrust to get enough pressure and takes time starting one engine at a time. Maybe they had an after start problem, but 30 minutes is really nothing to talk about.