<p>We report a numerical study of two-dimensional Rayleigh-Bénard convection in a square domain over a Rayleigh number range of 5 × 10³ ≤ <i>Ra</i> ≤ 1 × 10⁹ with a fixed Prandtl number <i>Pr</i> = 4.3. It is found that the compensated global transport quantities exhibit a non-monotonic Ra-dependent evolution from <i>Ra</i> ≈ 2 × 10⁶ to <i>Ra</i> ≈ 5 × 10⁷. Detailed modal analysis reveals that this non-monotonic behaviour is associated with the abnormal reversal frequency of the large-scale flow, featured by a pronounced drop and a subsequent increase in this regime, with a minimum reversal frequency occurring at <i>Ra</i> ≈ 2 × 10⁷. Flow visualizations reveal that this abnormal flow reversal scenario originates from the suppression of corner-roll growth via intermittent pinch-off events, which leads to long-lived unidirectional large-scale flow and critical slowing down of flow reversals. Vorticity balance diagnostics further show that this morphological shift in corner-roll dynamics is driven by a balance between buoyancy and advection at <i>Ra</i> ≈ 2 × 10⁷. These findings offer a new insight into the mechanism of flow reversal.</p>