Real life chemistry :boiling carrot dices
My guess would be natural convection. Hot water is less dense than cold water so when the heat source is at the bottom, the freshly heated water rises and the cooler water at the top sinks. The cooler water can't sink through the rising hot water, so the two flows run along side each other. Depending on the fluid properties, this can result in circular "rolls" of flowing water, known as convection cells. If you use a wide pot and small particles, you may be able to see many, but in a relatively narrow pot, you're more likely to observe a single recirculating flow around the centre, as shown in this simulation. The liquid loses more heat around the sides of the pot, so the descending flow rides down the sides and the ascending flow is in the middle where it can stay the hottest.

I'm guessing that your dice is too heavy to ride the ascending flow, but the flow from the edge to the centre pushes all the carrots towards the middle.