Hiya Joanne,
Ours do it too

We've noticed that our little short, fat, extra hairy Maisie pig doesn't lose hair but Rora (taller, longer, slightly less hairy) moults a fair bit - Shorty is definately more like Maisie in size, whereas Rora has the same body shape and hair coverage as Brother & Lumpy. I don't think there's any kind of link but just wanted to let you know you're not completely on your own - we have baldy looking pigs from time to time too

If their skin is not inflammed, red or sore/broken then I'd bet they are just moulting a bit.
Have a look for eggs as Dixie says, if you're still not convinved you could ask the vet if they'd examine a skin scraping, that should show up any kind of infestation.
Hope they're back to being hairy boys soon

Karen x