Corrections staff at Riker's island get unlimited, unauthenticated sick leave, which may partly explain the large number here, along with their employment as clerks, bakers, door control, etc.
Incidentally this is what I meant above, about the union being out of touch: unlimited sick leave makes staffing unpredictable, which makes work unpredictable and frankly dangerous, which makes guards take more sick leave in a vicious cycle. The union has weaker-than-expected motivations to fix this, because everyone with seniority does not actually work in the housing blocks.
Incidentally this is what I meant above, about the union being out of touch: unlimited sick leave makes staffing unpredictable, which makes work unpredictable and frankly dangerous, which makes guards take more sick leave in a vicious cycle. The union has weaker-than-expected motivations to fix this, because everyone with seniority does not actually work in the housing blocks.