Agree with Tiny. They can and do trip. Sometimes it's because they're not paying attention, but on autopilot if you will.
I've found some of ours sometimes do this baloney on purpose when we're leaving out, but on the way back, they're surefooted as a mountain goat...
And yes, it could be the rider not being neutral in the saddle, or a combination of all of the above.
If joint issues, like a bum knee, or hoof/farrier issues, are ruled out, I'd go with lack of paying attention and/or faking it to get out of work. If faking it, I doubt the horse intended to get himself banged up, that could have been completely by accident.... as in he faked a little too hard, if that makes sense.
Throw in a rider that might have been caught off guard by the stumble or didn't know how correct their weight distribution correctly in that split second, then yes, it could have taken the horse to his knees.
"We are here to laugh at the odds and live our lives so well that death will tremble to take us."