The reason why it foams is because blood and cells contain an enzyme called catalase. Since a cut or scrape contains both blood and damaged cells, there is lots of catalase floating around.
In the Injury Theory, it's damage to the arterial endothelium, followed by platelet activation, then smooth muscle cell migration to injury, then macrophages, with resulting "foam cells." Engorged foam cells burst, starting the injury cycle all over again
A. Martinez, B. Binks, и D. Langevin. Abstract Book of the XXIII IUPAP International Conference on Statistical Physics, Genova, Italy, (9-13 July 2007)