Final answer:
George was about to open what he presumed to be a mummy crate when he encountered a series of ominous signs that pointed towards danger, reflecting on the nursery's too-realistic and potentially hazardous African veldt environment.
Step-by-step explanation:
Just as George was about to open what he thought was the mummy crate, he had a series of startling experiences which indicated that something was amiss. The door shook as if something jumped against it from the other side, suggesting a sense of foreboding or imminent danger.
In the context of the narrative, which describes a high-tech nursery that can create virtual environments, George had also been reflecting on the too-realistic African veldt that the nursery had been creating based on the children's thoughts.
The lifelike experiences, including the sun's heat, the lions, and the smell of blood, had become overwhelming and potentially dangerous, leading to George's decision to lock the children out of the nursery, as he pondered on the risks of such powerful fantasies becoming a daily norm for his children.