Two things that I can think of bring about this scenario. 1.) Development of the individual raptor, i.e. imprint or parent reared, will effect their level of wariness. Imprints usually are aggressive toward other raptors and will rout them when they enter its airspace or leave their kill to rout them or carry their kill on approach by intruder and then offer it as a distraction while getting the upper hand and beating the interloper into submission. Parent reared birds for some reason do not seem to acquire this natural tendency and I assume because their development in the chamber somehow precludes the acquisition.
2.) Weight. Parent reared birds tend to have to be flown a little more on the hungry side than an imprint of the same make and model. As such, whereas an imprint might feel like this is its meal, the parent reared bird might feel like this is its life necessity. The parent reared bird therefore may be less likely to want to give it up, or be so distracted eating that it just doesn't take the oncoming threat with enough seriousness. Passage birds probably would also feel this way due to their hunger at flight time, but they have already gained some experience in the wild to understand that such an altercation may be lethal.
Hacking and tame hacking (with another bird at least) may help the parent reared birds but not many falconers can acquire more than one bird at the same time to be able to offer that. Some breeders offer parent reared birds that are also hacked, but I have also heard of those being killed more often than I would think would be satisfactory. But, even so, I think having a sibling or two or ten can really help for the young bird to learn to keep an eye out and always prepare to be blind sided...even if its all fun and games. Play prepares them for life.
When I tame hack my imprints, if I can do two I will, but if not, I help to convince them of threats if I can see them before they are getting lined up on my bird by mimicking the warning call of the species I am flying. Any sort of 'roc roc roc' or 'cac cac cac' call in rapid succession will at least alert them and they will suddenly start craning their neck to discover what has put me on alert. I suppose these days you could use your cell phone to broadcast a call in a pinch.