marble size eggs

[teedles24]

I have found an egg occassionally the size of a marble. What causes this? My chickens have been laying for a year.

 

[Den in Penn]

Welcome to BYC.
A glitch in the production cause the "fart" egg. They can happen with hens just starting to lay or hens going into a break, like molting. Sometime the glitch never goes away completely.