Did you fall forward or backwards? I couldn't quite figure that out based off your post.
If you fell forward chances are your edges did dig into the box/rail. Are your edges super sharp? Factory fresh or recently tuned, especially under foot? Sliding a rail or box on new edges is pretty scary. Sometimes you'll be fine, but sometimes that sharp ~90 degree metal edge will dig into the metal you're sliding across and cause you to fall forward, have your back foot stop, etc etc. The best thing to do is to de-tune the edges under foot. I use a diamond stone and gummy stone pretty regularly on my park skis to remove burrs and other sharp spots under foot that might catch.
This is a good time to mention that sliding a lot of rails and boxes will ultimately ruin your skis. It's a bummer, but that's just how it is. Metal sliding across metal over and over causes a lot of wear. You're edges will become rounded underfoot and eventually non-existent. You'll get cracked edged from landing on the rail or box, which can lead to de-lamination problems if moisture gets into the ski and then freezes. Even if that doesn't happen eventually you'll get so many edge cracks that actual pieces of edge will start falling off your skis. As soon as you start sliding rails essentially you're fighting a losing battle. It's super fun, but it's important to be aware that you're causing damage to your skis. I've seen a lot of people disappointed that their skis "broke" from sliding rails, and you really can't feel that way. No one warranties rail damage (with a few unsuccessful exceptions).
Back to what happened to you. If you fell backwards then you weren't weighting properly. Some keys to success are landing with a wide stance (at least shoulder width), with your knees bent, and with your weight evenly distributed over both feet. The weight distribution is often the hardest part for new park skiers. It often helps to focus on weighting your front foot a little bit more than your rear foot, as your natural tendency is to weight the back foot more, which will cause you to slip out and fall backwards.
Have fun!!