If the left main was extended all the way but not locked it should have been fine. The weight of the airplane alone should keep it from retracting. Those wheels fall 23 inches as they retract.
It takes considerable hydraulic pressure to force them to gear-down against the wind. If there's any sort of leak in the system, it might not get the mains down at all. If just the left main failed, it might be a broken gear trunnion. There's an SB on those; they're famous for cracking, and the hydraulic actuator itself has also had cracking problems. It splits at the rack roller and jams. These systems need more maintenance than they often get.
All speculation, of course.