How possible? Since it happened, it is 100% possible.
How probable? It depends on the type of slide mechanism, and the likely modes of failure for that type. Pretty easy if the mechanism has no locks and has hydraulics to leak or cables to break. Less likely if gears have to shear, but still possible.
I see a lot of slide outs going down the road open a few inches, creeping out slowly.
Most slide out systems that could fail disastrously get delivered with locking bars. Using adequate locking bars properly will make the even unlikely for any type of mechanism.