Snap ring (aka: c-clip) is none reusable part according to Toyota FSM. When snap ring reused, it no longer fits snug around axle in it's groove. Each time reused used, the snap ring expands a little more. This almost always means, it is the factory snap ring (2.2mm thick) being reused time and time again. Which almost always means the hub flange to snap ring gap is to wide (spec max 0.19mm). Very often the gap, is now 0.30mm or greater. This wider gap than, allows the FDS axle to move in and out of hub flange a greater amount than designs for. Does this matter, YES:
1) Pounding on snap ring, due ti excessive travel of axle. Increasing width of groove on axle. It also pound on brass bushing at rear of knuckle. Thus increasing gap.
2) Not, snug around axle. Means less metal for snap ring to be retained by. So eats away more easily at axle, and snap ring. This, enlarge gap and rounds the once shrap right angle of the groove in axle. So that, no snap ring can hold on end of axle.
3) The increased gap, also widens the space between the double seal assembly of FDS to knuckle. This can result in water entry into needle bearing and bushing.
4) Almost always with these snap rings that have popped off. The wheel bearing are loose. Loose wheel bearings result in chatter (vibration). This vibration and axle moving in and out of hub flange. Turns axle into a saw. Thus eating away at axle & hub flange splines (teeth).
Bottom line; it is from improper wheel bearing service. Which now due to not replacing a $2 snap ring. FDS, wheel bearings and likely axle bearing and bushing are damaged. Or about $900 in parts per side ($1,800) plus labor!