Because in 'Catdom' instincts are telling her that as the oldest cat, she's going to be under attack from the others trying to be #1. It happens in all cat hierarchies and there is very little you can do but try and protect her from the others. I've gone through it numerous times, and sometimes the best thing is to help her find a high place in a corner that she can easily get to, with bedding, where she can hold off the others when they've chased her there, and possibly in a room with her own box (probably won't last long as her own, but...) and after she uses the box, or after feeding, make sure they don't chase her (for some reason those are common chase times). Also use Feliway plug-ins because they should help, and just keep a close eye on things. Try to distract the younger ones if they take a bead on her (it's what they do) before attacking. It's one of the few things about cats that make me angry even though they're just being instinctive.