Okay, so ya boy Zagreus comes from an earlier offshoot of Greek religion called Orphism, believed to have been founded by Orpheus himself. Zagreus is actually said by some to be an earlier form of Dionysus. In this alternative version of the story, Zagreus is the son of Zeus and Zeus's own daughter, Persephone, whom the god seduced in the form of a snake (does this guy just have NO boundaries?).
Hera gets all jealous when Zagreus is born, so she sends some Titans to rip the godling apart. They distract him with some toys for a bit, but then Zagreus gets the whiff of danger. He runs away from the mean Titans, turning into a bunch of different animals to try and avoid them. Eventually, though, they catch him while he's in the form of a bull and rip him limb from limb.
After this, Zeus found Zagreus's heart, fed it to Semele, and that's how Dionysus was conceived. (If possible this alternate version is grosser.)
Chances are that the worship of Dionysus either grew out of the worship of this older god, or somewhere along the way the different cults merged together. Either way, we're pretty dang sure that the ritual dismemberment (sparagmos as the Greeks called it) of bulls was a fixture of Bacchic rituals. It was said to recreate the way that Dionysus himself was dismembered as a child by Titans and later reborn. This death by sparagmos and resurrection was a key part of the cult of Dionysus.
The myths of Dionysus aren't the only ones to include a little dismemberment, though. Lots of ancient religions practiced their version of sparagmos on sacrificial animals. There are also other gods who were said to have died by dismemberment and been resurrected. The most famous example is probably the big time Egyptian god, Osiris, who was said to have been torn apart by Seth. His sister and lover Isis hunted all over the world and put his body back together again, and later the higher gods resurrected Osiris as the Lord of the Underworld. (A gloomy post, but whatever.)