why not wait for the tap then press your ult?
if I had to guess, it is because he had no satchels to animation cancel. The only way to win for talon there is to push PRX, so if he waited for tap, he would be caught mid animation because talon would flood into main
edit: also, by ulting slightly early, he denies tap (and potential stick) AND push into main, buying his team valuable time, especially for mindfreak