וְהָיָה אִם לֹא תִשְׁמַע בְּקוֹל ה' אֱלֹקֶיךָ וגו' (כח, טו)And it will be if you do not listen to the voice of Hashem your G-d... (38:15)The Tiferes Shlomo says that the reason we recite the tochacha at this time is as a sign that the year and its curses should come to an end. If, Heaven forbid, a bad decree was issued against a person, we hope that it will be fulfilled by reading from the Torah about bad things, and he will then be able to experience only good.

This article from The Way Of Emunah explains that reciting the tochacha before Rosh Hashanah serves as a sign for the year's curses to end, allowing bad decrees

Let the Year and its Blessings Begin

Collected Thoughts on the Weekly Parshah From Rabbi Meir Isamar Rosenbaum shlita