Jack wasn't even thinking that he might scare her, he was just thinking of getting her out of there and the safest way, for both of them, had been what he had just done. And since his teleportation skills had been hit or miss lately - no doubt thanks to lingering after-effects of the genuinely kind Mrs. Butters' smoothies - he didn't dare put both of their weight on the already faltering ladder without knowing for sure it would be okay in the end. And he was right to not even attempt it, as within seconds the ladder and Krissy were both coming down.
As he had promised, he was right there to catch her as she fell - he was stronger than he looked - running from that spot as the ladder and whatever wall it had been leaning against tilted over and fell just after her leaving a large dust cloud in its wake.
"Are you okay?" he asked, looking her over when they were a safe distance away, just before he put her to her feet but still held on to support her if she needed it. He knew from their conversation earlier that Krissy was no damsel in distress and she would definitely not appreciate being carried more than she had to be. "Your foot, it happened so fast, but your foot looked like it was caught in one of the ladder rungs."