Homura is from the future, so she knows when, why and how the Walpurgis night will start.
I'm betting the key is Madoka. Incubator stated multiple times Madoka as an incredible latent power, and in that ep he even stated she'd be God like. Since he also stated at the end of the ep that Magical girls are baby witches, that means he feels Madoka will be an incredibly powerful witch, devastating the town or worse... hence the Walpurgis night.

Now, I'm also willing to know who Homura is.