Norwegian goes the American way...

That was a surprise this morning - it was long rumored that Norwegian is one of the airlines that already committed for the B737MAX (100 firm orders plus 100 purchase rights in addition to a follow-on order for 200 B737-800NG), in that case the -8 model. But that they also would order the A320neo (MoU for 100 aircraft plus 50 purchase rights) could not have been expected.
There are two possible explanations that come to my mind as reasons for that decisions to go away from a single-type fleet:

1. They went the AA way (of negotiating), meaning they negotiated with both Airbus and Boeing until they got a deal from both they could not resist to close them both.
2. Their anticipated growth is too large that one of the OEM's could deliver aircraft fast enough to satisfy that growth.

On the other hand it is a little bit discussable if a mixed fleet of B737-800NG and B737MAX-8 would be a single-fleet type anyway. The engines (CFM56-7BE and LEAP-1B) do not have anything in common and Boeing itself talks about 85% commonality between the NG and the MAX. So Norwegian would have had a two-type fleet anyway until the last -800NG would have been phased out. Now that Norwegian gets aircraft from both Airbus and Boeing the last -800NG will be phased out earlier so that the (at that scale) marginally higher costs through having a two-type fleet will be (probably) more than offset by the lower fuel costs.

The next interesting question will be the engine choice for the A320neo. Although the selection of the LEAP-1A to complement the LEAP-1B on the MAX-8 fleet would be the most logical choice, they maybe also go the AA way and buy from both engine manufacturers, like AA did it recently for their A319 (CFM56) and A321 (V2500). So the PW1100G could come into play - the factsheet that Norwegian provided with their press release shows the fan diameter of the PW1100G (81"), but that does not necessarily say anything...