The developers made a very correct decision for this beast and made a not very large radius of its aggression. If he would have a higher target radius (like a raptor, for example), he will start shooting for the first step, and it will be very annoying for all players.
NEVER UNDERESTIMATE THESE GUYS, they are able of dispatching your tamed velonasaur in seconds because unlike ridden velonasaurs. They can fire at a longer and more rapid rate. If your velonasaur is un-mounted, then it may very well be able to kill the wild velonasaur!
If you want to fight or knock one out, get up close and hope that it will then opt to use its melee rather than it's spines, as the latter will shred you in a matter of seconds. Trapping it with a bola is also a good method as it will be unable to turn its body to fire at you, but still be wary of it's AOE where it shakes and shoots spines in a small radius around it.