Each led has it's operational voltage and proper resistor has to be used to cut down the source voltage to the operational voltage.

If the led are easily burned, high chances the source voltage is not reduced with proper resistor. I believe the intention is to make the led as bright as possible.

However, please don't take my word as it is. Checking on the resistor value  used against the led operational voltage will reveal the root cause of the the led burned.

Cheers.