Lottsa ways and reasons the best can be taken down by the worst.
Being a former SEAL, Ranger, or Green Beret doesn't necessarily mean you know how to run security details or drive your way out of trouble; that is specialized training even they may have not had, or not had enough of. And even if they had, the lions may simply have been overwhelmed by too many jackals.
Their local intel, or their access to it, may not be as clear as our hindsight. Don't know enough about their route, or alternate routes to know if that made any significant difference. May be limited in how unpredictable they can be by forces not under their control...
May not have made any "mistakes" and just had bad luck, or may have known their situation was screwed up, not much they could do about it, did the best they could with what they had, and soldiered on anyway.
Those who dare don't always win.
"Names in my ears, Of all the lost adventurers my peers, -
How such a one was strong, and such was bold,
And such was fortunate, yet each of old
Lost, lost! one moment knelled the woe of years."