Why Does Theta Decay Accelerate as an Option Approaches Expiration?

Theta decay accelerates because the value of the option's extrinsic (time) value becomes increasingly sensitive to the passage of time as expiration nears. With little time left, the probability of a significant price move is low, and the option's premium rapidly collapses to its intrinsic value.

This rapid collapse is reflected in the non-linear, accelerating rate of Theta decay.