What is this abcess on my stags?

I got this problem in the 2014 season and once again this past 2015 season. About 1 out of every 5 Hatch/Kelso stags get it.

I don't think it affects their performance at the shows.


This is where they get it, right on the breast bone.


Let's take a knife to it.


Normal blood at first, I think.


Then you get these little balls.


A big one popping out.


Close-up on my hand.