They somehow managed to turn the stove on while warming up Ramen noodles in the microwave located on the counter on the other side of the kitchen. And as such an astute fireman would decide, the beer and plastic was placed on the stove earlier in the night.
What did they do when the alarms were going off? Well, Enron lived up to his name and took wet towels to the kitchen, but Burke went running around the whole complex finding each and every alarm only to stomp on them.

umpkin: