How could someone possibly die from drinking monster energy drinks?

Yes: Some of the chemical components of monster energy drinks could trigger fatal disorders in ill people.

Caffeine poisoning: Caffeine is a stimulant, and has a molecular structure very similar to amphetamine. Some people are highly sensitive to its effects, others not so much. Some people develop tremors, anxiety and insomnia after a single cup of coffee or a cola, others drink gallons without problems. SOme of the deaths associated with these drinks may have been in people with underlying cardiac problems.d

Arrhythmia: Death can arise from caffeine toxicity priming the heart for arrhythmia. There are reports of sudden death under investigation currently by the FDA.