Objects released into a granular packing close to incipient fluidization may float or sink depending on their density. Contrary to intuition, earlier work reported that under certain conditions, a lighter sphere can sink further and slower than a heavier one. In this study, we reveal the mechanism of this intriguing phenomenon using both magnetic resonance imaging and discrete particle simulation. Our findings suggest that local fluidization around the sinking sphere and the formation and detachment of gas bubbles play a critical role in driving this anomaly.