는 데 오다

come to where X happens

Two things that sound identical are mixed together here. 데 is a place, so 축제하는 데 오니까 is 'coming to where the festival is' and 노는 데 왔다 'came round to where we were hanging out'. Most lines, though, are the connective -는데, which just sets a scene before what follows: 근무하고 있는데 오더니, 'I was at work when they came up'. A space separates them in writing and nothing does in speech.