Just a speculation GeeRam ~ If he was banged out at 1000 ' had they been on a TFR flight or preparing for one it seems the plane flew on for quite a distance so it was trimmed for level flight so yeah a forcible ejection may be the cause.

Talking of fly away's,

In WWII from one of my books on the BoB a Spitfire was hit by 1 20mm shell which came through the port side of the cockpit and neatly removed the top of the pilots head as a hard boiled egg is done, he was listed as missing in action no one saw him go down.
The plane was perfectly trimmed for level flight and that's what it did until it settled nicely into some tree tops with little damage where it was discovered a month later with the pilot still in there and baked by the sun for that month with a fairly intact cockpit canopy.
They knew he was still in there as they could smell him climbing the ladder what they found and described is best left out but you can imagine the poor pilots body extremely decomposed.