Rashi: The simple meaning is, when Shmuel turned to go away, Sha'ul grabbed Shmuel's robe ¹ [and it tore], for he wanted him to return until Sha'ul will bow in Gilgal, where the Ohel Mo'ed [once] was.

Rashi (citing Opinion #1 in Midrash Shmuel 18:5): Shmuel tore Sha'ul's robe and gave him a sign; refer to 15:27:2:1.

Radak (citing Opinion #2 in Midrash Shmuel 18:5): Shmuel tore his own robe. ² Tzadikim normally tear [their own clothes] when their Neti'os ³ (ventures) do not succeed.

Rashi (from Midrash Tehilim 57): (Sha'ul asked who will receive his kingship.) Shmuel said, the one who will tear your robe, he will rule in place of you. This is why when Sha'ul realized that David tore his robe, he said "I know that you will be king" (24:21).

Malbim (28): Even though a Nevu'ah for bad can be changed through Teshuvah and good deeds, if an action accompanies the prophecy, it will not change. The action represents fulfillment of the matter immediately.