Mechanism: explanation of final step in synthesis of the thyroid hormones (T4, T3)

In the following mechanism for the synthesis of T3 (triiodothyronine) on thyroglobulin, what are the factors causing the last step to occur? Does it have something to do with the electron density of the intermediate and/or bond strength? (NB thyroperoxidase abstracts the electrons in the first step so I don't know if it's involved much in the last step)
[![Mechanism for synthesis of T3 (triiodothyronine)](https://i.stack.imgur.com/4tT6V.png)](https://i.stack.imgur.com/4tT6V.png)


