I never have noticed "particularly thick" lambs haveing this. But those low chested lambs that do not not have the right neck set , are prone to this. Aas well as short necked lambs, short hipped , short coupled lambs.
Mostly its a shower problem. Use the head to drive the lamb, instead of using a knee into its chest.
Pushing on a lambs chest with a knee, while trying to pull its head off , with your hands, will almost guarantee this .
And a lot of show people , have this style down.
The head and neck should be pushed ,not pulled.
And lamb needs to be trained to drive from the hip and butt.Hump back is bracing with its back, as well as other issues.
genetics as well in play. And those low fronted,high hipped ones will likley always look this way.