I suspect that Norwegian's choice of BHX is market-led rather than indicative of a fall-out with MAN. Look at the three destinations. Malaga, Barcelona and Madrid. All three are symptomatic of the market saturation problems which have afflicted Monarch Airlines so recently (flies AGP and BCN, served MAD briefly).
MAN-AGP is one of the most competitive routes out there. There is no shortage of capacity. I have no inside track on yields but I'd bet on them not being great viewed over 12 months. MAN-MAD is currently the preserve of Ryanair, and as LAX_LHR tells us just today, Iberia Express is set to join the fray. IIRC, Ryanair plans upto 10 departures weekly on MAD, Iberia Express plan 4 weekly rising to 7. History tells us that is plenty for the Madrid route; we want no repeat of what happened in the recent past when four carriers launched in the same season. MAN-BCN is another well served route on which you would rarely struggle to book a seat. Would it be healthy from an airline perspective to add another carrier on this route?
Sometimes we need to be careful what we wish for. Yes, NAX is a great airline and I'm sure we'd all dearly love to see them expand here. But route selection is important. We must avoid a scenario of bloodbath routes ex-MAN (as Madrid and Berlin once were) which result in carriers exiting the market believing they can't make money here. With BHX-BCN seeing two new entrants close together, I hope that route can still make money as it is. Certainly, I would be very concerned to see new entrants (whoever they are) pitching in on MAN-AGP/BCN at present, and two carriers on MAD appears sufficient to me. Remember as well … anyone who tries their luck on these three routes needs to take on an incumbent Ryanair and win. To steal a line from Hollywood, "Feeling Lucky, Punk?"