It's impossible to guess the guy's motives yet, it would hardly be improbable for a vet with PTSD or one of the others to attack muslims in rage or flashback but it's also not particularly probable either. Unless he just pulled over and attacked on psychotic whim he probably did some homework, can't rule out the 'assumed they were muslim, hates muslims' option but I'd hardly take it as a given at this stage.