I have had a couple of similar experiences to the ones others have reported:
classic mode, moderate speed (~5mph), comfortably cruising/carving and then....straight over the falls, trying to run it out, meet the pavement, quick shoulder roll, assess the damage.
I have done enough board sports for long enough to know when I am pushing it too hard (yep, got a few of those boo boo's too) and when something else is going on. The frustrating part is that its hard to recreate this.
But, I am working on a theory that I would love feedback on if you have had this issue. Also, I think this idea would explain why this tends to be more of a noob issue. This has only happened to me when traveling at a slow-ish speed (one that a noob would tend to be going). I think what is happening is that, at that speed, hitting a rock, bump, etc is creating just enough deceleration to tip me forward off of the board but that there is not enough forward momentum for the OWs gyros/motor to kick in and accelerate out of it. I think this would explain why as you progress in riding, this tends not to happen (because we tend to ride with just a little more speed). Others agree/disagree?
PS, before you jump on the "it will get better with practice" bandwagon, I am a comfortable OW rider on many terrains (pavement, fire road, single track, beach, etc) and surface types (mud, wet leaves, hard pack, gravel, wet pavement, etc)...I get it.