Why'd that happen?

[Nikola]

When Link stabbed Ganondorf at the end, it showed Zant's neck snap. Then Ganondorf died...
What does that symbolize?? >_>

[Vaari]

Ganon needed Zant to live. That neck snapping is probably symbolic for the breaking of Zant and Ganon's bond, thus killing Ganon, or his physical body at least.

[Snefix]

The odd thing about that scene is that it appears, from the background and lighting, that Zant is nearby when his neck gets snapped and Ganon dies.

I submit that what may have occurred was Zant dying, simultaneously, as Ganon was dying.  When we were seeing Ganon was actually his stronger having overtaken the physical representation of Zant and replacing it with a manifestation of Ganon's likeness.