Whenever I boil meats, I get this foam on top of the broth when it first begins to boil really well. Growing up, my Pop always had me skim the foam off, I still skim it to this day.
What the heck causes it?
I am sure that JJ will be able to expand further but the foam is predominantly protein from the meat that is being denatured by the boiling water which then floats to the surface. Usually this protein is dissolved and is held inside the cells but when you cut a piece of meat you will rupture many cells allowing their contents to be released. There will also be other types of cells that that are not locked up within the meat mass (e.g. red and white blood cells) and these are burst and their proteins denatured by the boiling water. In addition to the protein there will also be surface fats that will melt and float to the surface. The net result is the formation of the (often unpleasant looking) foamy scum.
Might not be the right person but as a homebrewer, we call it a "hot break". It's a coagulation of proteins and once they form their bonds, the foam will settle out. The same thing happens with almost any starch, Potatoes, rice...
I never worry about it but if I am cooking, I turn the heat down a bit until the foam settles down and it will not boil over.