I had the exact same thing two batches ago. As soon as I put my thief in, carbonation started bubbling out like crazy, and the sample inside the thief had a tall head. Same thing in the hydrometer tube.
It's normal. I can't exactly explain all the conditions that cause this to happen, but the basic premise is that the yeast is creating a ton of CO2 during fermentation, and if the conditions are just right a large quantity will absorb into solution. Nothing to be worried about in my opinion, my beer turned out fine.