Is there any significance to the opening of the sliding doors?

To me, they are a symbol of the protagonist's transformation into a bull. In many films I've seen cattle depicted going through "automatic" (at least not operated by them) doors (slaughter houses and bull fight rings). That also explains his death inside the elevator.

On Roger Ebert's website, the critic Seongyong Cho suggests it's the imagery of his "wound" being reopened, the one he has been running from his entire life. She also notes that it corresponds to the opening narration that states you can't run from the past. It catches up with you and you're *beep* I believe it first appeared when his old friend came back into his life.