At age 21, prescription amphetamines are probably the biggest risk factor.
Prescription stimulants are remarkably safe in sensible doses—they make the heart beat faster but do little to destabilize it.
The stimulant risk comes from cocaine, which directly interferes with electricity by jamming sodium channels shut.
They're relatively safe for any given individual, but overall they're still a huge factor in sudden cardiac arrest at this age.
A few minutes with PubMed turns up no causal relationship. One study did find a sudden cardiac death odds ratio of ~2, but the dose response curve was flat suggesting lack of causality.
That's what I want to know, this is scary stuff...
It is often related to hypertrophy of the heart muscle, at least this is a common cause in the sudden death of young athletes.