I don't suppose the farmer is present at every birth. It would seem better to get the lamb out of the birth sac and breathing asap or they could get more dead ones.
There is nothing very natural about sheep farming these days - the lambs are bred to be large and sometimes they get stuck in the birth canal.
The ewe can also get exhausted during a long labour and run out of energy to push the lamb out and/or tend to the lamb immediately after birth meaning its airways might not be sufficiently clear.
The lamb could be born 'breech' which means it is the wrong way round and the legs get stuck and the lamb cannot be born naturally. The farmer will then attempt to turn the lamb inside the mother so it can then slide out easier.