Cats teaming up on single elder one. Have lived together for years, no moving. Help!?

My family and I own 5 cats. One is 8 years old, two sisters are 4 and the other two are 5. Until finishing week, everyone was getting along fine next to no major disputes (they enjoy all be together for three years). But now, the younger cats are attacking the oldest one. She does not retaliate, individual runs away (into the tub and ontop of counters), but still the other cats persist for no root at all. We are concerned roughly this sudden fighting, can anyone backing us figure out what is going on?
Answers: I wouldn't see it as a dominance issue sense they hold been getting along in need this problem for over three years. Is the oldest one in dependable health? Other cats sense when one is weaker within health. So they want to wipe out those. Just a thought...maybe i'm completely bad track also.
It is a dominance thing. The younger ones are realize that they don't have to consent to the older one dominate them. You might try some catnip to mellow them out.

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