My guess is their flight leader explodes.
Developers' advice is to use only airfields without small hangars for bomber missions. However I continue to ask them to introduce separate bomber spawnpoints to avoid this problem. Please report as bug-report if you want it to be fixed here
http://forum.1cpublishing.eu/showthr...=19629&page=40
Practical solution is to uncheck "spawn parked" and let them spawn on runway or change an airfield. ...Or to place a new airfield object on top of the default one.