Hope all are OK. 'How could it happen'? Well, self ejection with 'rear' selected on the command eject or an un-latched 'Top Latch'.
If the a/c landed without it's canopy, and the 'rail' out- see 1. If it landed with the (Rear) perspex missing, and no 'rail' -see 2
Happened before, Civil JP??
I truely hope all is well.
Advo