It likely didn't open up. Hollow point or open tip rifle bullets aren't designed for expansion like hollow point handgun bullets, it's just an artifact of the manufacturing process.
When a rifle bullet enters tissue, gelatin, or water, the spin imparted by the rifling is no longer sufficient to keep it stabilized as it was in air. The bullet begins to yaw because the base is heavier. As the bullet yaws, the drag on it increases dramatically. If the impact velocity is sufficient, the drag is strong enough to break a piece of the bullet off. At that point, the parts become substantially less streamlined and break up even further. The fragmentation causes each piece to slow more quickly than the bullet would if it were still whole. That transfers energy quickly and creates a multitude of small cuts or holes in the tissue, which weaken it so that the expanding temporary stretch cavity exaggerates those tears.
