There are many reasons why an abcess can develop.
First off the abcess is basically an infection and subsequent immune system response to some type of foreign pathogen in the body. Your body will basically wall off the pathogens and you will get a collection of dead skin and discharge in the pocket that is formed.
You can get it from inserting anything that is un-sterile into your body. Since most non human grade gear is not sterile, it is simply filtered a few times it can contain foreign materials that are likely to induce and immune system response. Most like though the gear is not the cause, its a dirty needle or a dirty amp stopper or even your unclean hands or dirty skin at the injection site.
Short acting tests contain a lot of BA because they have a short ester and they will not stabilize unless there is a high BA content. And as ironmaster says, some labs will use a high BA content even on long ester tests just because they thing it will somehow make it more clean, or close to sterile.
When injecting sterile liquid under sterile conditions and abcess is rarely ever seen, even with high BA oils.