Why did Jebediah survive after jumping out of a plumeting rocket?

45 meters per second is safe speed for leg-first landing. Helmet is weird bouncy thing.

After some testing I learned that as of KSP 1.0.5:

1. Kerbal landing at over 45m/s can awaken Kraken. Kerbal can end up below surface or whole game can crash.
2. Going over 50 ends up in "poof". Kerbal dies.
3. Kerbal helmets are made of unobtainium. Landing at much higher speed can be made by bouncing on Kerbal helmet. Extensive testing of head-crash is believed to be source of Jeb lack of fear.
4. Holding ladder on EVA while rocket explodes during crash can save Kerbal as well, as each exploding part greatly slows down rest of machine. Unless you broke elusive "poof" barrier, that is.

Note: With mods it is possible to equip Kerbal with parachute.

As to why poor Jeb suddenly went poof: Kraken Bug. Probably floating point precision related.