Personally, I think if OSHA waited for a serious injury to get involved with any employer, not just Disney, then shame on them for being a day late and a dollar short. Their business is to ensure workers safety, not identify why they were maimed or killed. Yes?
I read that neither of the tractor operators were injured. However, if there truly was a brake failure the potential was certainly there and I wouldn't be surprised if OSHA did step in to sniff around. It IS the responsibility of the employer to provide safe working environments as well as properly maintained (read: safe) equipment. If there was a brake failure and nobody was injured this would be considered a near miss. Things OSHA might look for would be why the brakes failed on the tractor: insufficiently maintained, operator error, lack of sufficient training, maintenance logs, what hours the operators had been keeping, who maintains the tractors, etc. OSHA might also question if there was a backup braking system in place (like an E-brake on a car) and, if so, why that either failed or was not used (reverting back to lack of training, fatigue, etc). There's a million ways to look at this, really. Certainly OSHA would be within their duties to have a gander at this incident.