Greg:
While I'm from a smaller area - narrow aisle truck population wise; but having been in the lift truck business for almost 30 years: I have not heard of a person being ejected from a stand-up compartment.
My initial response is one or a combination of (a) the driver was traveling too fast, (b) turning a corner too sharp, (c) had his/her load (forks) too high, (d) travel with reach extended and (e) not fully within the compartment.
Uneven floor conditions may have been a contributing factor as well.
With a deadman switch working properly, it's meant to active and stop a truck pretty quickly. Some operators use this for a normal brake on the truck and coupled with any one or more of the issues I've raised above; activation of the deadman brake in a rear end swing situation could cause the driver ejection to happen more easily.
Bottomline: I don't know the circumstances nor your application, but in general terms, I would blame this on driver error.
Garry