It's important to remember the work the onewheel does when considering its limit for speed. It is constantly working to balance/accelerate according to the rider's position. If that work rate gets so high that it doesn't have enough power to support increased acceleration/balance (such as what might happen at high velocity) then the onewheel cannot effectively work to compensate for the position of the rider anymore.
In this way, if you are leaning forward at maxed potential, then the onewheel does not have enough power to work to accelerate/balance anymore than it already does and since you are leaning forward, you fall that direction as the onewheel doesn't have the power to stop you.
I suspect it is similar to the feeling of stepping on the edge of a cliff with your front foot. The further you lean on it, the more weight the cliff bares until the point where it cannot bare anymore and it just gives way.
I wonder though if they could design a ride mode, or option in the app, that would keep some reserve energy to kick in at max work loads to assist with a more gradual deceleration from the max work load. So rather than just give way like a broken cliff edge, it might be a gentler drop like a partner moving toward you on a teeter totter. It would still be dangerous but perhaps easier to recover from in the moment.