I've pulled off a horde momentum. I will put these up from time to time to give you a little challenge. Now its because a wind makes them smell your scent, but I won't do something like this every time there's a breeze, don't worry. Hordes can be attracted by loud sounds, strong smells or by the sound of Callers. It is important to think of two things at horde momentums:
First, make sure you have enough firepower before you take on a horde. Otherwise you have to escape if you want to see the next sunrise.
Second, no zombies will be visible until I say so.
If later on in the game you think the hordes come too often or too rarely, let me know.