You say she is a normal mother in other respects, and does let them feed. So I would say it is that they are hungrier than her milk supply; she lets them feed till it's all gone, they're still hungry so keep trying, she pushes them off. Normal, necessary behaviour - they have to learn that they can't keep pulling on her empty teats; if she let them do that she would soon be down with mastitis.
Either top them up so they don't remain hungry after feeding time, and/or get her out onto your very best grass and give her plenty of good cake too. Outside on grass she should make more milk, so they may get sated or more nearly so, plus there won't be hard lambing pen sides for her to crush them against.
I'm not saying there's never a ewe doesn't attack her lambs and harm them, even outdoors, but to me the key fact here is that she lets them feed but then starts butting them away.
I have long pondered that keeping sheep indoors to lamb, and penned when lambed, creates different problems to the ones it solves...
