The trainer had 10 years of experience, was not some novice making mistakes. The lion was apparently recently captured in the wild and apparently not sufficiently tamed and trained.
Very sad, but I don’t want to blame the trainer.
I am not saying that an animal trainer cannot do that, ever. Heck, the famous WWI ace Lufbery had his two lions, Whiskey and Soda, they were fully grown and followed him around (uncaged and unleashed!) like he was their mother.
Point is, he didn't know this one well enough (apparently) to judge if he could turn his back on it. Even people who know what they are doing can fall prey to familiarity and contempt. Happens with pilots all the time.
An adult lion captured in the wild is never going to be sufficiently tamed and trained. Even those bred and born in captivity never lose their natural instincts.