I am commenting on some of the speculations made in this thread. From the conditions that Eazy has described, it certainly can't be the temperature. The mains voltage again is definitely ruled out because the supply to the LEDs comes from the DC output of the SMPS which is normally regulated well enough. For the same reason, Dipdude's point about "rectified and surge suppressed" is a non-starter. Humidity can be an issue because the plastic encapsulation of commercial LEDs is not hermetically sealed.
LEDs are current driven and not voltage driven. The light output is effectively proportional to the forward current. So driving an LED or a series string of them with a constant current source produces the best results. You never should connect LEDs in parallel unless they are closely matched. Otherwise the LED with the lower forward voltage will hog the current, deprive the other one, and finally fail faster. A simple substitute for a constant current source is to connect your LEDs in series and put a suitable resistor in series to limit the current. You can connect this arrangement to the 12 V DC source. The series resistor is extremely important. The value of the resistor is calculated to limit the current to about 10 mA or perhaps 20 mA. This actually depends on your LED's specifications.
As an example, if each LED has a rated forward drop of say 2 volts at 10 mA, then you connect say 5 LEDs in a series chain with a resistor of about 200 ohms. That will ensure that the LEDs last a long time.
BTW, all my advice is based on LED technology as I knew it about 7 years back before I switched over to the software industry. I don't know how much has changed since then.
