Sheep do have familial ties so I can understand the mother getting into the fray but as to why - who knows? You'd have to speak 'sheep' to understand the perceived offence I guess. Keeping a closed flock might be the reason I've never seem this in 20 years, most aggression seems to come from the males here. Wethers can be coarse with lambs that get into their field and the tup knows he is boss of the wethers.
Ps; is Bexel a local name for what we call Beltex up here, Belgian Texel, or is it a different breed?