I've also heard it's fairly common. Motor homes are not that rigid to begin with and as they age, they loosen up. When the motor homes twists over bumps or with jacks, etc it's not that hard for the windshield to pop out. Also, as they are exposed to sun, windshield gaskets shrink and harden so they become more susceptible to it.
On a somewhat related not, I've heard that with 3 jack hydraulic systems, you always want to raise the front (of motorhome) first otherwise, it's possible to crack the windshield if you raise the back first and then the front.