Theophylact of Ohrid

At this the high priest fulfilled the custom (he tore his garment). For whenever some calamity or sorrow befell the Jews, they would tear their garments. In the present case the high priest tore his garment as a sign that Jesus had supposedly uttered blasphemy against God, and that thus a great evil had occurred. But it is fitting to say here with David: "They were divided, and were not moved to compunction" (Ps. 34:15). …

Commentary on Mark ↗

Leo the Great

But Caiaphas, to increase the odiousness of what they had heard rent his clothes, and without knowing what his frantic action meant, by his madness, deprived himself of the honour of the priesthood, forgetting that command, by which it is said of the High Priest, He shall not uncover his head or rend his clothes. For there follows: Then the High Priest rent his clothes, and saith, What need we any further witnesses? Ye have heard the blasphemy: what think ye?

Catena Aurea by Aquinas ↗

Bede

But the high priest, rending his garments, said: Why do we still need witnesses? You have heard the blasphemy. The same fury that had shaken him from the priestly throne provokes him to rend his garments. He rends his garments to show that the Jews had lost priestly glory and had an empty seat of the high priest. It is also a custom among the Jews, when they hear something blasphemous and seemingly against God, to rend their garments. We read that Paul and Barnabas did this too when they were honored with divine worship in Lycaonia. …

On the Gospel of Mark ↗