The liquid shooting upward from the pit of your vortex may be explained by at least two effects.
The first effect was indirectly addressed by Albert Einstein in a 1926 paper he wrote on the erosion of river banks (http://people.ucalgary.ca/~kmuldrew/river.html). This effect is also called the tea leaves paradox, from an experiment Einstein performed in a tea cup. He found when he stirred the cup that tea leaves gathered in a little pile in the center rather than toward the sides, where they might be expected to migrate due to the centrifugal force.
Einstein reasoned that angular velocity of the vortex against the sides of the cup was slowed by friction, diverging from the general increase of velocity with increasing distance from the center. He also reasoned that angular velocity at the bottom of the cup is less that at the top, due to friction against the bottom. The velocity gradient at the sides from top to bottom causes the vortex contents to migrate downward at the sides and into the center, forming a circular flow perpendicular to the plane of the vortex and depositing the contents of the vortex in the center.
Thus, there's a tendency for the bottom of the vortex to leap upward in the center as it tries to complete its circular flow from top to bottom to center and back to top. See figure 1 in Einstein's paper, linked above.
The second effect is caused by the tendency of gases to become less soluble as temperature increases, and by the fact that the center of an irrotational vortex tends toward singularity - it rotates slowly or not at all.
You said that your wort was near the boiling point. At this temperature, gas starts to vaporize out of the liquid when it finds points of nucleation. The rapidly swirling outer reaches of the vortex do not allow gas to accumulate at nucleation sites. But as the center point is either stationary or moving very slowly, it provides a nucleation site for gas bubbles to form.
Coupled with the tendency of the vortex contents to leap upward at the center due to Einstein's circulation, these bubbles may propel spurts of liquid straight up, which explains the phenomenon you observed.