What would cause a post op infection in the Achilles tendon?

Surgical risks. A risk of any surgery, albeit small, is infection. It is more common to be caused by the skin flora you already carry but can be caused by others, especially if a dressing or cast becomes soiled before full healing of the skin has been achieved.
Same as any surgery. Some times people get post op infections. There are a few different reasons how or why this could happen. There is normal bacteria on skin and they can get inside the incision site and cause an infection, sweat can cause an infection. One can develop a stitch abcess from the suture material or even be allergic to the suture material. These are some ways.

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Help please? How long does a staph infection last post op Achilles tendon debridement?

Depends. Was the staph infection before or after the debridement. Antibiotics will usually clear it up in 1-3 weeks depending on the depth and structures that are infected. Read more...