Thankfully you are OK. A couple things that come to mind is:
1) Is it possible that you were close to or over the 300 pound mark? I know you say you weigh less, but with gear and everything, being a few years old maybe you just overloaded it? My Lone Wolf Sit and Climb is rated for 350 pounds. Gather up all your gear that you take, and step on a scale with all of it, see just how close you are/were to that 300# limit.
2)I broke 1 stand on a real knotty tree. Sometimes you get on a knot or bump in the tree and it twists the stand a bit, instead of distributing the weight evenly, weight shifts to one side and bends or breaks. When mine broke, I was on the way down, taking rather large steps.The deck hooked into the tree a little crooked and I dropped my weight down on it and it bent slightly. It wasn't until the next climb that it failed completely and it looked a lot like the stand in the pictures.
3) I kind of relate tree climber weight limits much like rating weight limits for a rope. A rope might be able to hold a load of 300 pounds on a consistant basis but take that same 300 pounds and drop it from 2-3 feet up. The 300 pound rated rope will break and may take up to a 1000lbs rating to stop 300 pounds on a 2-3 foot free fall. Take a 250 pound man and have him jump up and down on a 300 pound rated climber, I would expect the 300 pound rated climber to eventually fail. Add a tree that's not perfectly round or straight, it's pretty easy to stress out A climber. The entire climber industry is geared to make the lightest climber possible. The lighter the climber, the less they'll hold. I weigh 160 lbs and a 350 lbs climber is perfect for me. If I weighed 250 I'd want a climber rated to 400-500 lbs. When buying/using a climber, better make sure it fits you. A 250 lbs person, plus his cloths and gear, in a 300 lbs climber, it's just a matter of time before the climber is going to fail.