I'd guess it's a challenge for a change in the pecking order - if it were linked to their seasons I think you'd have had problems before now.
Who was top pig before, was it the older one? If she's a bit ill (or has the limp) the younger ones will sense this and could be fighting it out for the top spot. I'd separate the limping one from the others until she is back to full health - you HAVE to treat her though, no matter how stressful it is. You can't just let it go on as it might get worse and worse and no treatment is ever as stressful as having an animal dispatched and collected by the knackerman

I'd go for antiinflammatory jab and long lasting antibiotic jab first - that way you don't have to go near her with a needle for at least 5 days (give her a chance to forgive you

) but if they're all in together she'll be getting a hard time, so split her out until she's completely recovered.
If it's any consolation, head/ear injuries suggest a minor piggy disagreement rather than 'proper' fighting where they'll attack each others back ends and bite at vulvas - so it should all settle down once everyone is back to full health.
Good luck and keep us posted!