I added about an ounce of pellets to around 1.3 gal of fermented wort and this happened.
There was very little head space when I added the hops. It was up to around the handle someplace. It foamed like this for around five minutes or so and then died back down.
The fermentation ended about a week and a half ago and it's been sitting in the secondary for almost a week already. Hopefully it's just excess c02 coming out of suspension. I hope I don't need to post this in 'post your infection'