Why does voltage regulator heat up and starts to switch the supply when it is used for driving a high power LED?
Your DC/DC converter can deliver a continuous current of 1.5A, and 3A peak.
Depending on its configuration (for 6V or 12V), the LED can pull more than that, and nothing is keeping it from doing that because you haven't put in a resistor to set the LED current.
What probably is happening is that the DC/DC converter overheats because of the high current, goes into thermal shutdown, cools off, starts up again, etc., making the LED blink.
You must put in a resistor to set the LED current. Calculating the resistor value requires the datasheets and knowing if the LED configured for 6V or 12V, and to what output voltage you have set, or want to set, the DC/DC converter.