The four kayakers in an overloaded kayak were already unstable. Entering into this fast moving water, the current may have thrown the bow off the path of the kayak's forward momentum, causing it to upset. Or the current could have grabbed an upstream paddle, dragged it under the kayak and flipped the boat that way. Tilley Point forms a promontory or headland. The current will be flowing its strongest here, creating chaotic waters.
Winds are also stronger around headlands increasing water hazards.
Knowing how to read the water would have indicated to the paddlers that something was happening. Eddy lines and upwellings are clearly visible phenomenon. Avoiding them is the safest choice for an inexperienced kayaker. After their rescue the Americans commented on the speed of the response. They were fortunate.