Why does bacon, and specifically, the fat that's released, foam/h when you pan fry it?

Every time I fry bacon in a pan, by the time I'm halfway through a batch I can't even see the bacon anymore from all the foam. I'm curious why it does this. I can't seem to find any information about it, but one site vaguely implied that 'real' thick cut salt-cured smoked bacon doesn't do this, but the vacuum-sealed kind that's prepared in brine (the usual stuff you grab at the supermarket) does.