The lattice QCD studies indicate that the critical temperature T-c similar or equal to 260-280 MeV of the deconfinement phase transition in quenched QCD is considerably smaller than the lowest-lying glueball mass m(G) similar or equal to 1500-1700 MeV, i.e., T-c much less than m(G). As a consequence of this large difference, the thermal excitation of the glueball in the confinement phase is strongly suppressed by the statistical factor e(-mG/Tc) similar or equal to 0.00207 even near T similar or equal to T,. We consider its physical implication, and argue the abnormal feature of the deconfinement phase transition in quenched QCD from the statistical viewpoint. To appreciate this, we demonstrate a statistical argument of the QCD phase transition using the recent lattice QCD data. From the phenomeno-logical relation between T-c and the glueball mass, the deconfinement transition is found to take place in quenched QCD before a reasonable amount of glueballs is thermally excited. In this way, quenched QCD reveals a question "what is the trigger of the deconfinement phase transition ?".