There is no initial catalyst, it's a time loop.

Fry did the nasty in the past-y.

Thus giving him a brain without a delta brainwave.

Which then made him invisible to the brains. So nibbler chose him to sneak into the brains head quaters to plant the bomb destroying the brains.

To ensure this would occure, nibbler pushes fry into the cryopod at the crucial moment. Also future fry is there cuz time travel reasons.

Sounds about right last time I watched it that was the so-called origin story of why and how Fry was trapped.