Poor sanitation can be one cause yes. Poorly sanitized injections can lead to infections in the abcess that spread to the body.
I think what the OP was talking about was a sterile abcess, which is an abcess that doesn't result in infection(fever, headaches, etc). These can happen from pinning day after day in the same spot (from chemicals in the gear), from injecting gear too close to an artery or lymph node, or from injecting a tiny piece of the rubber stopper as well.