Sources close to the "Shooter" star tell us he was off-roading back in July when the utility task vehicle rolled over, so Ryan and a stranger tried to flip it back on its wheels. Problem was -- the parking brake wasn't on, so when they turned it upright it started rolling downhill towards a crowd.

We're told Phillippe rushed to grab the UTV's steering wheel and steer it to safety, but it was moving too fast and yanked him underneath ... causing the full weight of it to run over his right leg and crush it.

As we reported ... Ryan gave a thumbs-up update of his injury shortly after it happened, explaining it was a freak accident and not work-related.