The pressure obviously built up overnight as fermentation commenced. I sure didn't expect to find this with 2 gallons of headspace in the 5 gallon carboy (that is a 3 gallon batch for those not good at math). I guess the yeast was healthy and obviously began with an active roar.
krausen, blocking it up. I've heard of this happening but this is a first to experience it. I pulled it out of the top stopper to an audible release of pressure. I can only imagine what a few more hours (or minutes) could have done. Boom, with krausen and wort splashing all over the place.
Lesson of the day: Use a blow-off tube for initial and active fermentation. Don't assume you have enough head space.
Here is the photo of it now happily fermenting away.