That has to be it - now on the other hand if they were drinking...
From the U.S. Department of Justice's National Criminal Justice Reference Service (publication NCJ 145534): "Of all psychoactive substances, alcohol is the only one whose consumption has been shown to commonly increase aggression. [...] Marijuana and opiates temporarily inhibit violent behavior [...] There is no evidence to support the claim that snorting or injecting cocaine stimulates violent behavior. [...] Anecdotal reports notwithstanding, no research evidence supports the notion that becoming high on hallucinogens, amphetamines, or PCP stimulates violent behavior in any systematic manner."