Jet2 has an ageing fleet and need new(er) aeroplanes. In today's market of cheap money, and the opportunity no doubt to screw Boeing on price in order for them to fill production slots before the introduction of the Max, what would be a more cost effective way to obtain new/additional aeroplanes? Aeroplanes cost money and airlines need aeroplanes.
With some of the order to replace existing airframes, expansion is significant but manageable in my opinion. With the exception of the seasonality of the business and it being UK centric (which need addressing), Jet2 are in a far stronger position than some of it's competitors. The BHX base is no doubt intended to heap more pressure on Monarch.