Help What is this big cave occur of reason?

Toast

 

country bread cave

Why occured cave?

Feedback please.

It's called a "flying crust".

Possible causes:

1. Overproofing

2. Insufficient gluten development (weak flour / not enough kneading)

3. Not enough tension in the outer skin after shaping

It does not look overproofed, so I would guess a combination of #2 and #3. Good luck!