Granular clogging at narrow constrictions is commonly described as a memoryless Poisson process, capturing the stochastic onset of clogs but not the emergence of mechanically stable arches. Using discrete-element simulations of a vibrated two-dimensional hopper with repeated clogging events, we reveal a two-stage mechanism of clog formation: random Poissonian triggering followed by a finite structural-consolidation process that stabilizes the incipient arch. Tracking competing outlet-spanning chains further shows that the eventual final arch separates from transient alternatives only during this interval, through reduced geometric defects and strengthened weak contacts. Although consolidation durations vary strongly across events, their evolution is confined to a narrow directed trajectory in a state space defined by obstacle packing fraction, particle velocity, and effective coordination number. A graph neural network trained solely on particle-contact graphs independently recovers this ordering, showing that instantaneous configurations encode the consolidation state and clogging probability. Thus, clogging is random in time at initiation but predictable from configuration after structural consolidation begins.

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• Granular flows
• Granular packing
• Granular materials