Easyjet are an extremely prudent airline, they are very focussed on the shareholder and returns. They usually do not start large battles unless they are confident that they can win. They will shift capacity rather than get into a blood bath and lose money. They will pick on smaller rivals to wipe them out, like AerLingus at Gatwick, but with Norwegian who have lower costs, they take a wait and see approach.
The truth is the level of competition has significantly intensified. Ryanair are going onto many busy routes that are long standing busy easyJet routes eg STN to EDI and GLA. The two have coexisted on many routes and got on with it but FR are going into routes at busy bases like MXP, starting at Belfast etc etc. Vueling are very clearly intent on making a ply for easyJet territory. In a show of strength easyJet has to make an investment to deter Vueling. EasyJet technically should be adding lots of capacity at LTN to fill the gaps, otherwise Vueling week be all over it in no time...