Okay so my friends and I have been investigating this
I will name 18 possibilities to how the Zombie Apocalypse will start:
1.VooDoo
2.Mad Scientists
3.V.R.V. Vrykolas Romeros Virus(book)
4.Brain Parasites
5.Neurotoxins
6.The Real Rage Virus
7.Nuerogenes
8.Nanobots
9.A Mad-Cow Diseased Hamburger
10.Cordyceps
11.Chemicals&drugs
12.Drug Mutation
13.Hemophilia Mutation
14.Evolving Dementia
15.Texting
16.Toxicmosa Gonddi
17.Prisons
18.Rabies Virus
Please add more

I'd like to expand my list!