The building is only like 4 years old - so I doubt there's anything spooky going on...
Seriously, I'd think mosnab is right and she is freaked about biffing it again there. But also, could the puppy have wet the rug or something, and the big one knows that's a no-no, and is avoiding the rug?
The dog did sprinkle a little bit on the rug the other morning when we'd just woken up and were getting dressed to take him out. But he's had accidents all over the house and she hasn't been scared to go anywhere... Maybe this one's lingering a little bit since it was in the rug.
I think theory number one though is the slipping by trying to jump before the rug, because I'm pretty sure I saw her do that recently and there's no telling how often it happens when we're at work.
Last night she went through the danger zone when I got home and they were both jumping up and down excited to see me.
Then later that night she wouldn't again.