It's Alive!

What's going on in my oven right now?  I am currently baking a batch of Guinness brownies, for St. Patty's festivities tomorrow (more on that later, depending on how they turn out) but I just checked on them and here's what I found happening:

a large, unsightly mound in my brownies?!
Judging by the batter, they are going to taste fabulous, but seriously.  What is happening?  Is it just the yeast?  I've made beer bread before, which of course rises in the middle, so maybe it's just something I've never noticed before.  But scary.

UPDATE: 5 minutes later.  I have just taken it out of the oven and the big bulge has now cracked on top and steam is pouring out.  It looks like a creepy old swamp man blowing smoke.  eek!  Will definitely frost these brownies.