Why does NaOH catalyze H2O2 decomposition?
According to THE HOMOGENEOUS BASE-CATALYZED DECOMPOSITION OF HYDROGEN PEROXIDE J. Phys. Chem., 1961, vol. 65 , pages 304–306:

$\ce{H2O2 + OH- &lt;=&gt; HO2- + H2O}$ 

$\ce{H2O2 + HO2- -&gt;}$ [6-membered ring with 2 "H"s bridging 2 "O-O"s, with another H bonded to one of the "O"s] $\ce{-&gt; H2O + O2 + OH-}$


  It is clear that the rate will go through a maximum
  when the peroxide is 50% ionized, that is
  $[\ce{H2O2}] = [\ce{HO2-}]$.


So OH- catalyzes decomposition of peroxide, but a maximum is reacted when the amount of hydroxide added is half the amount of peroxide.  If more hydroxide than peroxide is added, the catalytic effect is lost.