Why do eggs rock/roll and then go quite ?

[rosiethechicken]

Its day 19 and while I am waiting on my hatching, I am wondering why some eggs rock/roll and then go quite ? One of my eggs was rocking a lot on day 18th, but its quite now.

 

[stoopid]

It's hard work breaking out of their little jail.
Probably just resting, they will start up again.
As soon as you aren't looking!
Good luck on the hatch