Don't know.

In my case it was obvious why it happened. My antenna was partially down, and the offending TX was close by with the antenna up.

In the real world you would never expect this to happen. You would have to have a fade of some kind from the "interfered with" transmitter (bad transmitter antenna orientation, weak or out of tune tx module, etc.) and the interfering tx might have had an ideal antenna orientation, strong module, right on tune with the rx, etc.

To be honest, when I first heard about this (a few minutes after it happened), I thought it could be elevator servo failure. I guess David has ruled that out.