From personal experience, I'd say that its your awareness of almost falling asleep that is startling/exciting you back awake.
This is very hard to control but nonetheless, can be. I dont know whether your WILDing or just trying to fall asleep, but this sudden awareness can snap us back awake in either case. The reason is, we're not supposed to be conscious when we're sleeping, therefor, when emotions like shock (sudden awareness) or excitement are present, this tips off our body that we're not unconscious yet.

How do you get past these emotions? For WILDing, I'd say practice. Practice is the only way of getting used to feeling your body slowly or suddenly begin to fall asleep without reacting to it in a way that's going to wake you.
This can also help even when you're not trying to WILD and you're just trying to fall asleep regularly. But if you find this is too much of a hassle, then you should sleep w/ a fan or something like that for you to focus on and calm your mind. This helps to fall into unconsciousness.